FACTORY STAMPS ------------------------------------------------------------------------------------------------------------------------------------------------

Every back label has a 2-digit number stamped into the upper left corner, ranging from 00 to 28. Most games can be found with multiple different stamps, usually between variants, or even sometimes on otherwise identical copies. These stamps have nothing to do with the game itself and, when talking about an NES collection, do not represent a set of variants. So what are they? The answer is actually pretty boring. These stamps are a quality assurance tool used by the manufacturer to tie that exact copy of the game to a specific factory and/or assembly line. So if there was ever an issue with a batch of games they could trace it back to the source, hopefully figure out what went wrong, and fix the problem. This type of system has been in use forever on all types of products from electronics, to cleaning supplies, to food. 

The stamps can be used as a tool to make an educated guess as to whether-or-not a back plate has been switched out. For example, if we look at 100 copies of Ninja Gaiden with Oval TM SOQ and REV-A 2 label, and they all have a 19 stamp, then it's reasonable to assume that a copy with an 11 stamp is not original.

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For cartridges manufactured in Japan there are 15 unique factory stamp numbers, pictured below. The 04 stamp is extremely rare. Early copies of games have numbers that are more rounded and larger. Later games have stamps that are taller and skinnier. There is no significance to this other than the fact that the dies used to make the stamps must have been replaced at some point.