i recently noticed that my mario’s tennis carts have a small impression towards the bottom right-hand corner of the label. My US carts have the number “19” impressed as does one of my JP carts. However, the other JP cart has the number “12”.
i checked my other retail carts and they also have numbers.
galactic pinball 09
v tetris 22
panic bomber 12
red alarm 09
I believe carts for all Nintendo systems have similar imprints. I don’t know what the numbers mean, but apparently if there’s a letter (usually A) after the number, then that indicates a later revision of the cart.
In some cases the presence/absence of a stamped number can help you tell the difference between an authentic cart and a bootleg.
Edit: looks like the prevailing theory is that the number indicates the factory where the cart was manufactured. It’s a way for every cart to be “identical” but still be able to track down where the cart was made. For example if people are having problems with carts that are all stamped with the same identifier, then Nintendo can check out that specific facility. Interesting!
You’ll find such numbers on many items, such as the bottoms of aluminum beverage cans. I’d be willing to bet that the stamped numbers on Nintendo game cartridges indicate the production line on which the cartridge was manufactured.