I found the following references, though none are supported with documentation.
This seems to contradict the advice given on your other post about the UBI Zenith cue ball separation methods. @Jermey told you:
The ball return uses a magnetic cue ball to separate it and make it return to the correct area.
...but this other reference said (emphasis ours):
United was built up in NY, and went out of business a long time ago. The cue ball is separated by being over-sized. The coin mechanism had an arm on it that, when pushed in, tripped the latch that holds the ball plate in place, causing the object balls to drop into the ball return tray, followed by a returning of the spring which locks it up again.
I think it depends on the age of the pool table. It sounds like they might have switched over at some point. If you see a big magnet (or electromagnetic mechanisms), then that's clearly the method. If there is some kind of passageway that detects size (e.g. an over-sized cue ball) then that's likely the method.
Another reference I found mentions:
It used an over-sized cue ball, but the separation took place in the removable ball box, which looks like it's missing.
UBI Ball Return Patents
Also, United Billiards, Inc. was Art Daddis' company. I think most of UBI's pool tables were made to capitalize on patents Art was granted for his developments in pool table ball return systems, and others. We should be able to dig up the Art Daddis patents for some rough drawings and explanations on how some of it works. He was part of (or worked for) a few other pool table manufacturers e.g. so some of his patents would have been used by others, but they might help.
If it comes to that, I don't mind helping to look them up for extra detail.
For example, here's a diagram of one of ball separator / ball return mechanism patents held by United Billiards, Inc.