Does it reject quarters when coinmech is build in the coindoor?
Coinmechs tend to give problems due to the angle of the pinball machine. They kinda lay against the door.
I also had this problem and added some bushings on the topside of the coinmech the straighten them up. Works fine now.
About that cast part, i also don't see any use for it. Maybe, if you know the manufacturer, there could be some manual of this mech.