payout completes because search index 17B opens and disconnects the start/replay reset circuit for duration of the payout.
most likely anti-cheat relay switch E is connecting wire 56-2 to 75-2 ... manual page 418.
since all the wires on that switch probably look grey, just check switch E and make sure it's doing something sensible. 56-2 is connected to the middle blade, and should only touch the blade with 75-2 when the anti-cheat relay is unpowered, so 75-2 should be on the top blade. If necessary, stick paper between the contacts and see what happens.
if the game is on free play, you may need to check how they did it ... tho I guess you said "insert coin", so mebbe it's not been modified.