I've identified a Frogger cart that I own whose ROM is one byte different to all of the others that I have. I've examined the ROM to see if it's a difference in any of the graphics but that doesn't appear to be the case (the graphics appear to all be stored much earlier in the rom). So, I can only assume that the one byte difference is a tiny change in the code/data but it is well beyond my skill set to be able to disassemble the roms and figure out what the difference is and why. So, I'm loo
If the chips have a standard part number (and if you actually want to open the cartridge to find out), it would be interesting to see if that top level chip is really a 4K chip. If so, you'd see the exact copy of the lower 4K of that chip simply because it has 4K worth of address lines but it still sits on the 8K select line.