Posted by wayout440 on another thread, very useful info:

The least flexible CPU boards is system11c, since the sound circuit is completely removed, and a separate sound board is used. This board can only really be used in system11C games because of this. Populating the missing sound section on a system11C board would be difficult, as some of the sound chips are hard to find (and expensive). But the system11c CPU board *can* be used in many sys11/11a/11b games if the accompanying D-11581 sound is used too.
The most flexible CPU board is system11A. The system11A CPU board can be used in any 1986-1990 sys11, sys11b or sys11c game The only exception is if the sys11A CPU board is going into a original sys11 game (High Speed, Road Kings, or Grand Lizard). In this case, connector 1J15 (four pins, .156" molex male header) must be added to the sys11a CPU board (that's all that needs to be done). To use a system 11a board in a system 11c game, don't use connectors 1J16 (volume control) and 1J15 (speakers). Also the sound ROMs at U21 and U22 are not used, since they are moved to the sound board.
The original system11 CPU (as used in High Speed, Grand Lizard, Road Kings) is also very flexible, after the CPU board is modified slightly. It can work in any sys11, sys11a, sys11b or sys11c game with these modifications. CPU connector 1J18 pins 6,7 must be run to ground, otherwise the special solenoids will not work in sys11b and later games. Also there are six 1N5234 6.2 volt zener diodes that should be added to the back of the CPU board. If using a system11 board in a system11c game, just don't use CPU connectors 1J16 (volume control) and 1J15 (speakers), and the sound ROMs at U21 and U22 are not used.
System11B CPU board are also very flexible, and can be used in any System11A or 11C game. However the system11B CPU board will not work in the four original System11 games (High Speed, etc.) though, because of the missing amplifier circuit. System11B will also work in a System11C game - just don't use CPU connectors 1J16 (volume control) and 1J15 (speakers), and the sound ROMs at U21 and U22 are not used.

The System11b CPU board is not quite as flexible as System11a or System11, because 11b is missing the U1 sound amplifier and associated parts. The System11b CPU will work "as is" in a system11a or system11c game. But these sound part must be added back for a System11b CPU board to work in a System11 game (High Speed, Road Kings, Grand Lizard). If using a system11b board in a system11c game, the CPU sound ROMs at U21 and U22 are not used.