I have not previously looked at the invoice & return numbers in Stout but on a quick look those raise a number of questions. As you note, it appears that Cuautitlan returned 100 of the six centavo 72s to Mexico City. Other subs of Mexico returned earlier stamps to Mexico City. However, if I am reading the records correctly, it appears that Mexico City returned a total of only 62 six centavo stamps to the GPO (including all years). Stout, p. 6. So what happened to the other six centavos from Cuautitlan that were returned to Mexico City?
Possibilities: (1) Mexico City used them or reissued them to other subs. Could yours be from Tacubaya? Sz 851B? (2) The records are incomplete or incorrect. (3) The "idiot" hypothesis. (4) Something else that I cant think of.