Model  System Board  Description

VIC1541 1540001-01   Very early version, long board.
        1540001-03   As above, only the ROMs are different.
        1540008-01   

1541    1540048-01   Shorter board with a 40 pin gate array, Alps mech.
        1540048-03   As above, but Newtronics mech.
        1540049      Similar to above
        1540050      Similar to above, Alps mechanism.

SX64    250410-01    Design most similar to 1540048-01, Alps mechanism.

1541    251777       Function of bridge rects. reversed, Newtronics mech.
        251830       Same as above

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    Short board, new 40/42 pin gate array, 20 pin gate 
                     array and a R/W hybrid chip replace many components.
                     Uses a Newtronics drive with optical trk 0 sensor.
1541C   251854       As above, single DOS ROM IC, trk 0 sensor, 30 pin 
                     IC for R/W ampl & stepper motor control (like 1571).
                     
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, single DOS ROM IC.

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.