25^6=244,140,625 - I think I have seen estimates of 100,000,000 addressable entities in the Japan postal address system (a bit half of them being households, the rest companies,public buildings, various organizations and so on )

TFA says "seven-digit combinations of numbers and letters".

Yes absolutely. But my comment was just to confirm what you were saying about 25^7 (with one control digit) i.e. 25^6 would be enough to cover everything that is addressable in the system today. If we increase that to alphabet+ numbers 35^6, it is even more feasible.

Oh, I see, thanks for clarifying.