Mexico Airmail Set

I bought a small collection from a local here in town and found this in one of the envelopes.




In the envelope were receipts from the Ecuador and Argentina Postal administrations for purchases of Stamps. There was no receipt for this one. Question, was this an "Official" souvenir ? Or is this something perhaps put together by a local Stamp dealer ? The 1928 lettering must be some type of serial number and not the year since some of the Airmail stamps are the 1931 - 1932 issues.

I can think of at least two possibilities...

The vignette of the airplane (the Mexico Excelsior) also appears on the 1928 issue, with the portrait of Capt. Carranza in the place occupied by the eagle/serpent symbol. Perhaps a bit of catalogue blindness was at work; or

Auction lot number. Perhaps an unlikely coincidence to be one number off from the year of issue for the majority of the stamps...?

The early Mexican airmails are very sharp.

Or it could just be back dated too and not necessarily had to be dated the year of release as the top Stamp, center row is dated 1931. Is that the one you're referring to ?

Yes, they are a beautiful set of stamps,
I love early Mexico,
their art deco designs were magic.