Closest schematic I have is an 873, and it shows there are at least three 31 wires.
They are all separate circuits, btw.
One of them is to the coin lockout coil (behind coin acceptor),
One of them is to the odds reset coil (not the relay, but the solenoid)
The 3rd is to the jackpot lockup and to the Key switch JP release...
Hopefully you can find it on those coils etc.
Schematic shows coin lockout coil going through a coin lockout relay switch to a 30 wire (the return)
I am not familiar with a coin lockout relay, so your guess is as good as mine.
I am guessing that it is to your coin lockout coil (behind acceptor), and to check that, after inserting one coin, push on the armature to feel if it has voltage to the coil. It should buzz at you and you can feel resistance. If it loses voltage after 1st coin in, then check switches on your odds disc in the top, the ones operated by the tabs poking out from the white wheel.
Is that original switch NO or NC, by the way...?
Happy New Year.