it's possible they added the pinball score reel to replace the mustang unit.
I'm not sure what the game play is supposed to be. I vaguely recall something about a horseshoe win will enable a bonus payout opportunity on the next 10 spins, in which case the standard pinball score reel with 10 positions would seem useful. The top glass picture will help.
the contact plate on the score reel is wired to connect a circuit at 1 of the 10 positions assuming none of the traces are jumpered together on the not visible side of the board.
if ya can figure out what the score reel is doing, the game may function correctly once ya find a problem with the circuits someone created.
the top relay is the regular J.P relay. Not sure where the green wires on the score reel are going or the wire colors they attach to. If you want to email highest possible resolution pictures of the game to slotpics@cdyn.com, that'll help.
the schematic for a 918-1 is on
https://bingo.cdyn.com/slots/ ... it has credit play, which it looks like your machine used to have and someone removed the replay register. Is there a button on the outside of the cabinet near the top of the handle?