Interesting. I'm confident of the RLGL ROM dump, since I got the same results visually and electronically. From the COP4xx User's Guide, it doesn't look like there are any differences in the instruction set as compared to the other COP4xx chips, other than removal of some opcodes and interrupts.
I assume with Einstein that they programmed the game with a COP410 in mind but for some reason wound up using the COP421. I guess it's theoretically possible that they wanted some functionality that the '421 has that the '410 doesn't, but I can't think of what that would be for such a simple game.
At some point we'll probably run across a TMS-14xx/-16xx that uses the latched inputs, and maybe even the frequency divider.