What changed with WPC-S and WPC-95
WPC-S (World Cup Soccer on) adds a game-specific security PIC chip at U22 on the CPU board. WPC-95 (Congo on, plus some Jackbot games) has its own CPU and driver board, folds the Fliptronics circuit into the driver board, combines sound and dot matrix control into one audio/visual board, and puts the security PIC at G10.
Systems
Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)