Thanks guys. The only difference between the CPUs is that the one on the expansion board is an MC68HC000P8 whereas the Mac one is just MC68000P8. Might try to reverse engineer the board a bit later, but the presence of the board makes the entire thing hard to disassemble (the mainboard doesn't slide out, I have to pry the board out from the bottom :O

Forgot to mention that the double-row header just to the right of the board's 68k connects via ribbon to a DB-25 on the back of the Mac. That's odd too.