So looking at that, it's not a simple switch for the trigger. There's a signal off the PCB in the gun. So no way to test if it's the gun or the board. However, I think it's the gun board. I tried a different IO board with the same results. Just not sure what else I can try given what I have on hand.
So revisiting this, despite not currently having a working CRT in my cabinet... I found a fully gutted system, with original Sammy guns for cheap enough. Wired it all up, and it's working. So now I just need to determine if the Happ guns are faulty, or the wiring I used, or the gun-board, etc. Fun! Well not much fun on my little 9" test bench PGM.