Model  System Board  Description
1540    1540001-01   Long board.
1541    1540001-03   Only the ROMs are different.
        1540048-01   Short board with a 40 pin gate array (Alps).
        1540048-03   Short board with a 40 pin gate array (Newtronics).
SX64     250410-01   Most similar to 1540048-01.
1541A    250442-01   Alps or Newtronics drive selected by a jumper.
1541A2   250446-01   A 74LS123 replaces the 9602 at UD4.
1541B    250448-01   Same as the 1541C, but in a case like the 1541.
1541C    250448-01   A new 40/42 pin gate array, a 20 pin gate array
                     and a R/W hybrid chip replace many components.
                     Uses a Newtronics drive with track 0 sensor.
1541-II              A complete redesign using the 40 pin gate array
                     from the 1451C and a Sony R/W hybrid, but not the
                     20 pin gate array.

NOTE: These system boards are the 60 Hz versions.
      The -02 and -04 boards are probably the 50 Hz versions.

The ROMs appear to be completely interchangeable.  For instance, the
first version of ROM for the 1541-II contained the same code as the
last version of the 1541.  I copy the last version of the 1541-II ROM
into two 68764 EPROMs and use them in my original 1541 (long board).
Not only do they work, but they work better than the originals.