Ted McElroy didn't use different model names to distinguish the major changes to his cast-iron bug until mid-1937, when he introduced his "new 1938 Mac Key." But before that, there were five distinctly different Mac-Keys, plus one major variation, plus many minor variations. I have, therefore, taken the year-model designation style that McElroy used for the 1938 model and applied it to those keys which he failed to differentiate by a model name. And just as McElroy's "1938" model appeared in 1937, my year-names aren't intended to be an accurate reflection of the actual appearance of the key. Their purpose is simply to permit a handy nomenclature for collectors when discussing a particular style of Mac-Key. Also, while he initially called his bug a "Mac-Key," he dropped the hyphen with the 1936 model.

The information presented here is based on that in my 1993 McElroy book, but has been updated to reflect more recent information. This page is a work in progress. I will add photos and descriptions of the various models from time to time.

All Mac-Key weights should be drilled to fit a round 3/16" diameter pendulum. The weights are brass. The earliest Mac-Key weights are unplated; some in 1935 were chrome plated, some following ones were cadmium plated, and pretty much all of the later weights were chrome plated.
-- Early Mac-Key weight (1934 to early '36 with split connector bar): 1" diameter, 3/8" thick.
-- Mid Mac-Key weight (1936 to 1939 models): 7/8" diameter, 5/16" thick.
-- Late Mac-Key weight (A-400, P-500, S-600): 3/4" diameter, 1/4" thick.
These sizes are typical for the years and models stated but not always strictly followed.

1934 model In late 1934, Ted McElroy came out with his first telegraph key. This bug is shown here in the ad from the December 1934 issue of QST magazine. Not many were made as McElroy soon added an improvement to the design (see the 1935 model). He then "recalled and changed over about 100 [1934 model] keys" (R/9 Magazine, February 1935). It is more likely that he traded the owner a new 1935 model for the 1934s and then either modified or destroyed the earlier models.

1934 model Here is a photo of a 1934 Mac-Key. It can be readily distinguished from later Mac-Keys by the cantilevered bar holding the fixed dot contact. A small clip at the damper post (see below) can be used to lock the pendulum and the key can be turned on its left side and operated as a straight key; this feature was continued through the 1936 model.

The cast legend found on the bottom of the 1934 - 35 - 36 models identifies the early keys as a Mac Key, and its maker T.R. McElroy as the world's fastest radio telegrapher. The very top line (obscured on some castings) indicates that a patent was applied for. That may be so, but one was never issued. The date 9-34 appears on all of the first three models, causing some to think that the later '35 and '36 are a 1934 bug. On the 1934 and early 1935 models the abbreviation in the center line was "GAD" (for guaranteed). Late '35 and all '36 models have the lower pivot screw accessible from the bottom, creating a hole and eliminating the last letter, and the abbreviation appears as "GA."

This is the cast brass lever pivot assembly found on the 1934, 35 and (shown here) early 1936 models.

The pendulum clip on the 1934 and 1935 models was a small metal tab held to the damper support with a screw. The clip on the 1936 models was wire (shown here on a 1936A with a special extensible pendulum for slow speed). Click on the photo for another view.

The nameplate used on the 1934, 35 and early 1936 models measured about 7/8 x 2-3/8 inches, and was brass.

1935 model Within a month of initial production, McElroy modified the 1934 model to give greater rigidity to the fixed dot contact by adding another cast post, and connecting the fixed contacts with two bars forming a Vee where they meet at the pivot frame. The resulting 1935 model is shown above right. Shown is an early 1935, with "shallow Vee" bars; on later 1935 models the bars met lower on the pivot frame upright resulting in a "deep Vee" variation.

1936A model The 1936 models are identified by the U-shaped connector between the fixed dot and dash contacts. On the very earliest 1936 model, the connector bar is in two pieces (two L-shaped bars joined at the center to form a U) as shown here. I call this variation the "split U-bar."

1936A model The connecting wire terminals on the earliest split U-bar 1936A bugs. They are conveniently located on the right side. Unfortunately, this feature would be gone when the one-piece U-bar appeared. Only about a half dozen of the early split U-bar 1936A bugs are known.
1936A model This is the later, common style of U-bar found on 1936 Mac-Keys. It is one piece. The wire terminals were now far apart, one on the damper post and the other at the front left U-bar mount.
Sometime early in the 1936 model run the small nameplate became aluminum, as shown here.

1936B model: type 2 pivot. The late 1936 Mac-Keys (1936B models) have their lever pivot formed from steel bar stock. There are still separate pivots for the dot and dash levers.

1936B model with terminals McElroy finally realised that the location of the wire terminals on the Mac Key was inconvenient, so he started putting them on the base of the late 1936B models. He also made them available to Mac Key owners who would make the modification themselves on earlier bugs.

Junior This model used sheet metal for the base and pivot/contact support. It was identified by a decal on top of the pivot support (which is often faint, or missing). The Junior first had the type A cast pivot, and later the type B steel bar pivot.

Navy model Until the mid-1930s it was U.S. Navy policy that "Such 'bug' type keys as there are in use throughout the service are the private property of the individual radioman." Whether it was an earlier inquiry from the Imperial Japanese Navy for an order of Mac Keys, or the publicity McElroy generated from that, in 1936 McElroy was awarded a contract to provide speed keys to the U.S. Navy. The military designation is CMK-26009. The design, with its new damper style and having the wire terminals on the base, begins to resemble the popular Vibroplex keys. Cast into the left side is the legend "PROPERTY OF US NAVY" (this was probably a contract requirement, since the Bunnell Navy keys also have a cast property identification).

Navy nameplate This large-size steel nameplate on ther CMK-26009 used McElroy's given name, Theodore. It was, in many respects, similar to the plate that would soon be used on the 1938 and 1939 models.

1937 model The civilian version of the Navy key. Starting with left-over Navy base castings, the small aluminum nameplate was affixed, and "U.S. NAVY" was ground off so that the legend reads "PROPERTY OF." Presumably the owner would scratch his name or call letters into the blank area. There are four known variations in the cast legend on the 1937 model:
1. PROPERTY OF remainder ground off.
2. PROPERTY OF remainder cast blank.
3. Side cast blank.
4. MFD IN BOSTON MASS USA.
See the serial number page which will give an idea of the scarcity of certain 1937 variations.

1937 model: PROPERTY OF. The most common civilian version of the Navy key is that on which "U.S. NAVY" was ground off so that the legend reads "PROPERTY OF".

On a few of the "PROPERTY OF" keys, that is the only lettering that was cast; the remainder of the original legend was cast blank. (Sometimes the grinding on the common variety was done so well that it takes a sharp eye to spot this scarce variation.)

1938B Deluxe model McElroy simplified the casting for his 1938 model, eliminating the cast damper assembly, and he used a new, large steel nameplate. (The nameplate identifies the model as either Standard or Deluxe.) All 1938 models came in Standard and Deluxe varieties. First came the Deluxe, which sported a circuit closer, and a "marbleite" finish on the base. The early 1938 models (1938A) still have the type 2 double pivot lever assembly. The later ones have a single pivot for both levers (type 3) as found on other bugs of the era, and as shown here.

Another feature of the 1938(A and B) Deluxe bug is the "dot stabilizer." This pre-tensioning finger comes in two varieties: an early large style, in which the block carries both the dot spring and the tensioning finger...

And a later thin style, in which the block carries only the tensioning finger.

For more information on the "dot stabilizer," see this page.

Left-handed 1938B Deluxe model While a left-handed 1936 model had its own casting, a later "lefty" was simply a modified Mac Key: The cast base tabs holding the fixed contact connector bar were knocked off, contact posts were installed, holes for the new damper location were drilled, and the nameplate was put in a slightly different location on the base. One way to tell a factory modification is that the original nameplate holes were never drilled, as seen by the locator dimples underneath the base.

1938B Standard model Six month after introducing the Deluxe, McElroy advertised the Standard model. This is finished in black crackle, and has no circuit closer or dot stabilizer. By the time the Standard came out, the separate pivots for the dot and dash levers were eliminated in favor of a single, Vibroplex-style pivot lever (thus the "B" designation). The dash lever has bent lugs and shares the pivot with the main (dot) lever.

Note that the 1938 Deluxe is found with both style pivots (that is, A and B varieties), while the 1938 Standard model is almost always found in only the B variation. Again, see the serial number page for details.

1939 model This was the last Mac-Key to carry the large nameplate. It is very similar to the 1938 model, but the T-bar or pivot frame risers are much wider, having a "hump" that extends beyond the upper horizontal top bar.This was available in two versions, the Standard, with a black wrinkle finish, and the Deluxe, in Marbleite (shown here). The Deluxe came with a circuit closer, but typically do not have the dot stabilizer (a few early ones do have the thin stabilizer).

No. 500 and No. 600 models.

A-400 model. This was a short-lived (and resultingly scarce) interim model between the earlier "T-bar" Mac-Keys and the P-500. The A-400 still has a pivot frame much like its predecessors, but without the extension tabs on the horizontal bar. And for the first time we see simple posts holding the fixed contacts. The Mac-Key is beginning to take on the appearance of its more successful rival, the Vibroplex.

P-500 model. Finally, McElroy's bugs became similar to everyone else's semiautomatic keys. The base and frame are cast and finished in black crackle paint. This is a good, useful bug with nothing special about it except for one thing: the McElroy name on the nameplate.
Cosmetic changes were made when McElroy offered this model to the Army Signal Corps. Originally, it had a black wrinkle base and chrome frame. Then the base acquired a smooth gray finish (the pivot support frame was still chrome plated). Themodel name had changed to CP-500, and then later to model 500-742. (In keeping with models of other 1940s McElroy apparatus, the "742" likely means July 1942.)

S-600 model The S-600 model, the "Super Stream-Speed," was the bug McElroy said he'd never make. It is "very poor practice to have a chrome or nickel plated base on a key," he wrote in a March 1938 flyer. He must have remembered this comment when he came out with the S-600 because he said the chrome had "a bluish tinge to prevent glare." This key comes in two models, the S-600-PC with platinum contacts and the S-600-SC with silver contacts. It also comes in three variations of mounting holes in the base: none, two, and three.