Mr T, it's quite likely the motherboard in your Mac is the original, though upgraded to 512K. The original boards were shipped in 128Ks even after the Fat Mac 512K was introduced. Apple apparently had a lot of the older boards on hand. You would be correct to use either motherboard in your Mac. By the way, the code name for the Mac Plus was "Mr.T".
JDW, I've forgotten the difference between the 64K ROMs. It was something very minor to do with the ROM debugger IIR (we were hoping for a fix for >512K of RAM of course). There were also multiple 128K ROMs, going by the Low-High names of Loud-Harmonica, Lonely-Heiffer, etc. and the difference there was mostly in handling SCSI boot protocol. The first Plus ROMs wouldn't boot a lot of drives, but disk drive manufacturers couldn't count on a customer having the fixed ROMs because Apple didn't do ROM upgrades.
JDW, I've forgotten the difference between the 64K ROMs. It was something very minor to do with the ROM debugger IIR (we were hoping for a fix for >512K of RAM of course). There were also multiple 128K ROMs, going by the Low-High names of Loud-Harmonica, Lonely-Heiffer, etc. and the difference there was mostly in handling SCSI boot protocol. The first Plus ROMs wouldn't boot a lot of drives, but disk drive manufacturers couldn't count on a customer having the fixed ROMs because Apple didn't do ROM upgrades.