I did a quick modification to the CPS1 multi filter board to add a connector for P3 and P4 button 3. I know it's used for Muscle Bomber Duo, but I don't know what else off the top of my head. The P3/4 button 3s are taken from the JAMMA harness P1/2 button 4 (pins 25 and c).
While you could just do it by swapping wires around in your control panel, I found this to be easier since my cabinet link is already set up for 4 buttons per player. I wanted to try to repurpose pins on the P3/4 or CN6 connectors but I decided against it since it would end up not being compatible with any real harnesses out there. I think adding a header to the filter board for this and the spinner would be a nice addition if there's ever a revision to the multi.
I also made a harness to support my cabinet link which I described here: https://www.arcade-projects.com/thr...-4-players-and-improving-2-players-too.23782/
Muscle Bomber Duo manual: https://www.arcade-museum.com/manuals-videogames/M/Muscle Bomber Duo Ultimate Team Battle.pdf
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