This part is the plastic ring that sits inside the plastic bracket that has outer gear teeth that the generally cylindrical decagon unit inner gear part rolls along. The purpose of the ring is to be the base for the spring between the score reel frame and that inner gear part. For pre-1975, the plastic bracket was metal and the analogous part was a similar ring, but without having the ability to stay aligned with the bracket the way the 1975+ ring can.

Here is the schematic from The Pinball Resource, but it doesn't list the part. The part I am looking for is directly below this spring.

http://www.pbresource.com/springkit/kt-gstep-05.jpg

I wouldn't be interested in buying a whole score reel just to get this dinky part. Cross my fingers, I think I have found this piece that I had lost, and can install it and not have to worry about buying one.

Also, this particular part is only on 1975+ EMs.