Maximum number of credits appears to be 1, but ROM and the DIP switches theoretically support max of 5, 8, 10 or 15. Not just 1. It would be good to understand why this is happening.

This is quite possibly related to the accumulator problem and possible RAM problem discussed in https://flipfix.theflip.museum/problem-reports/75/

http://www.pinrepair.com/sys1/
https://www.pinwiki.com/wiki/index.php/Gottlieb_System_1