It's a projection unit that keeps track of the credits earned and projects them onto a frosted glass, probably in a corner of the playfield. See one from a '39 Bally Champion at: https://www.funwithpinball.com/resources/1939-Bally-Champion#ProjectionUnit
It's basically a heavy duty step unit with a bulb, lens and mirror. There may be a credit knock off switch under the cabinet too.
/Mark