Can someone help me date 2 different editions of the Rand McNally Imperial Map of Mexico, please?

The maps look identical, but have slightly different printing codes at the bottom of the cartouche:
A-531600-215  -2-2-6  (the code is in black, except for the first -2 in red and the second in green). Based on acquisition info this should be 1970 or so.
A-531600-215  -2-2-2  (the code is in black, except for the first -2 in red and the second in green). Based on acquisition info this should be late 1960s?

Apart from that code, the population figures given in the right and left margins differ.

e.g. Mexico City is given respectively as:
(*9,000,000)  2,902,969
(*6,500,000) 3,150,000 ('65 est.) [so this map must be 1965 or later]

And Districto Federal as:
6,874,165   [which seems to be the 1970 census population]
6,073,100

Thanks

Brendan Whyte
National Library of Australia