Aha, I wonder if maybe that board is from some kind of redemption game. Seems like a board like that would cost too much for a home game, let alone a handheld.
Have you dumped any Rockwell chips, especially the PPS4/1 MCUs like the MM76EL? The data sheets mention a test mode to verify ROM contents, but I haven't found any details. When I have some time I'll try activating the test pin and see if I get lucky and it just starts spitting out data, like the COP444L.
Also, do you have any info on Rockwell calculator chips? We've been trying to figure out the Mattel handhelds, which seem to be B6001 and B6100 series chips, but haven't found anything but a few mentions in magazine articles and a couple of patents. I bought a couple of calculators with Rockwell chips, but they both are PPS4/1s, and the B6001 is much simpler.