The first three airletter cards in the correspondence, posted through October 1939, were copies of MEPSI ALC2 (Higgins & Gage = H&G FA2) with an illustration of the General Post Office in dark green on the reverse. The heading at the upper left consists of three lines of text and there is no return address corner card. The next four were copies of ALC2c (H&G FA2c) with an illustration of the Pyramids of San Juan Teotihuacan in dark green on the reverse. The eighth airletter card is an ALC2 posted in August, 1940, and is the only card not addressed to the primary addressee L. O. Williams, but to an associate at the Botanical Museum. Part of the correspondence appears to be missing as the ninth airletter card, an ALC2c, was written twenty months later, on April 7, 1942. As Figure 1 (HIRES) shows, this card has a boxed purple censor handstamp "PASSED BY US 2093 SA EXAMINER".
We now skip to the eleventh and last airletter card in the group which is shown in Figure 2 (HIRES). It is a commercially used copy of the so-called essay, "ALCE3" (unlisted in H&G). The heading at the upper left consists of two lines of text "CORREOS-MEXICO / TARJETA CARTA" and an ornamental line. The opening instructions are now at the bottom and a 7-line vertical return address corner card is at the left. The inscriptions and stamp are printed in a dull grayish green. Careful inspection shows that the frank has been reengraved since ALC2 (which was printed from engraved plates). On the reverse is the illustration of the Pyramids of San Juan Teotihuacan but now in a dull orange color. The paper is manila instead of the buff of the ALC2 varieties. The heading and frank on the front have been shifted to the right to make space at the left for the return address corner card.
This airletter card was posted "MEXICO D. F." on October 20, 1942. The contents are totally non-philatelic and refer to their mutual interests in orchids. Two additional copies known to me were sent by unrelated correspondents and posted "MEXICO D. F." on December 21, 1942 and January 25, 1943. The existence of these three used cards leads to the conclusion that this was not an essay, but a short-lived printing most likely made available sometime between April and October of 1942. Henceforth, this will be referred to as ALC3, not ALCE3.
The most intriguing item in the group is the 10th airletter card shown in Figure 3 (HIRES). This card has the same heading, frank and opening instructions as ALC3. The color and size of the format is exactly the same. The frank engraving is also the same as that of ALC3. On the reverse is the illustration of the Pyramids of San Juan Teotihuacan also in dull orange. The paper is a slightly duller manila than that of ALC3.
However, like the first two issues, this one has no return address corner card. Furthermore, since space is not required at the left for the return address, the frank and heading are shifted more towards the center as on the earlier issues. A possibility that must be considered is the presence of a smaller, three-line return address corner card printed entirely on the missing selvage.
The complete 7-line vertical address corner card of ALC3 can be seen on the mint copy shown in Figure 4 (HIRES). The top three lines, which have the text blocked out, might possibly have belonged to a smaller address corner card. Since the shift of the design to the left would have put the top line at or just beyond the outer edge of the selvage, the presence of such an address corner card is extremely unlikely. Furthermore, the writer put the return address on the back of the airletter card whereas he did use the vertical address corner card on ALC3.
This letter card was written on April 9, 1942, and posted "MEXICO D. F." on April 10, 1942. There is a censor label #2226 on the bottom selvage with brown tape on both sides. This letter has philatelic content as it mentions four new stamps that are being sent, but makes no mention of the new letter card itself. The previous letter card in the group, written and posted only two days earlier on April 7, was an ALC2c (see Figure 1 (HIRES)). The fact that "new stamps" were enclosed on April 9, would tend to indicate that the "new" card had been obtained at the post office along with the stamps and was being immediately posted. As mentioned above, the final airletter card in the correspondence, an ALC3, was sent about 6 months later.
Following the discovery of this variety, more than 50 years after its use, another copy was located and is shown in Figure 5 (HIRES). The entire heading can be clearly seen as an adhesive has been removed. This second copy was also postmarked "MEXICO D. F." some 4 months later on August 4, 1942 and is from an unrelated correspondence. This writer also put his return address on the back of the air letter card.
To conclude, this airletter card (suggested designation ALC2A) had the revised engraving, two-line heading, repositioned opening instructions and the dull orange reverse illustration of ALC3, but did not have any form of return address corner card. It is likely that it was available for only a few months during 1942. Please check your copies of these airletter cards. This author would greatly appreciate hearing from anyone who can add to our knowledge of these issues.
My thanks to Gerald Winerman for his comments and for the loan of items from his collection.