Sorry if this is late, but I think others might benefit from this information too.
After a closer inspection inside some of my carts the ROM chip has the first two parts of the code and some number, possibly version (usually 0).
For instance, my Mariokart Super Circuit GBA has:
AGB-AMKE-USA on the label and,
AGB-AMKE-0 on the chip
The 3-letter country code is missing, but the previous letter (at least for GBA games) actually indicates the region (E = America, P = Europe & Australasia, J = Japan, etc). I have a limited number of GB cartridges, but the ones with 4 lettered game codes seem to follow the same system, but the games with only 2 have a different rule. I only have two such games:
DMG-ML-CAN becomes DMG-MLA-1
DMG-YO-UKV becomes DMG-YOX-0
So all I have so far is A = Canada and X = United Kingdom (maybe).
If you have a larger collection you can probably work out the region from that. I hope this is helpful!
EDIT: more info about specific country codes here.