There may be a broader question that belongs in the "Main Stamp Collecting Forum", but my question fits here for now.
I checked out from my local library the Scott Standard Postage Stamp Catalogue 2022, Volume 2A C-Cur and have been identifying stamps from Colombia. Two of the stamps marked as "Aereo" I could not find in the airmail section of the Scott catalog: Bicentennial of the Birth of Francisco Santander, Colombia $190, and 500th Anniversary of the Discovery of America, Colombia $230.
I knew that Colombia issues particular commemorate stamps or set in both regular postage and airmail forms, and the main part of the Scott catalog will reference the number(s) of the airmail version(s). Extrapolating the probable year of issue from the theme of the stamps I went back to the main section of the catalog, and behold:
Bicentennial of the Birth of Francisco Santander, Colombia $190, issued 4/2/1992 Scott #1047, with the note that Nos. 1047-1047A are airmail.
Discovery of America, 500th Anniversary, Colombia $230, issued 10/05/1992 Scott #1062, with the note that Nos. 1061-1063 are airmail.
Why were these airmail stamps not listed in the "BOB" airmail section with a C before the catalog number?
I also noticed that there are no recent Colombian airmail stamps listed "BOB", C907 11/27/98 being the last entry in the 2022 catalog. Perhaps some countries are no longer issuing separate air mail stamps as most "first class" regular postage will often or usually travel by air, but the two examples I gave from 1992 pre date the 1998 last airmail catalog entry.
I have noticed this with a number of Latin American countries - starting in the 1970s, it seems Scott decided to assign regular numbers to air mail stamps that were part of a larger set, without the "C" prefix.