i've yet to find an "official" proof of what the double digit code is. can't be that hard to find an ex nintendo guy in japan or even usa or europe who knows that... until then, it's all speculation, though we're pretty close... would really like to have this confirmed one day.
here's a personal guess, but it's based on the front/back cart shell code, which supposedly follow the same concept (without A or B obviously):
i've opened many carts of the same region, and they all have 3X and 3X for the shell code (like F-33 and B-36)
so what i'm thinking is that the first number is a factory, and the second number... an assembly line inside that factory? only specific number are used for the 2nd number, for example 0 1 2 never show up.
exactly same logic for box inserts, i've done some checking on my wii inserts, and the same few numbers always come back for games of the same region, which most likely indicates a location - not a date for sure, since games from launch and 2/3 years later can have the same code.
also, the existence of 00 codes shows that they used hexadecimal base, but it seems they never needed to go higher than 9 anyway