Which System 11 CPU boards swap between generations
A System 11B CPU is the most flexible: it runs System 11, 11A, 11B and 11C games. In an 11C game, leave its volume (1J16) and speaker (1J15) connectors unplugged; its U21/U22 sound ROM sockets go unused. An 11A board also works in System 11 games. An 11C board only works in 11C games because its sound section is gone.
Systems
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Source: Repairing Williams System 11 Pinball 1986 to 1990, Deel 1, Clay Harrell (pinrepair.com)