Well, the service manual says:
J180,J181 ROM Compatibility Jumpers
A2000/500 ROMs: 1-2, 1-2
A3000 ROMs: 2-3, 2-3
A look at the schematics reveals a bit more:
The 2-3 setting ("2M") connects A0 to A17 for both U180&181 (pin 9 to pin 1). The 1-2 setting ("4M") connects A0 of U180 to 5V and A0 of U181 to GND.
The way this might be supposed to work: you need two identical ROMs in both U180 and U181! U180 is the low word and U181 is the high word ROM and that way every other word is skipped - pretty brilliant trick I'd say.
SCSI and 32 bit fast RAM won't work of course - A3000 magic isn't present in A500/2k ROMs (maybe 32 bit RAM can be AddMem'ed, but I doubt this because Ramsey isn't initialized properly).
