Alright, spent some more time with this machine today. To answer your question, Mr Phil-Lee it is a "Professional Home Model." I'm pretty sure when I looked up the serial number once before it was a 1972 model. It's a 606-1000 series. Any information on it would be appreciated.
Today I found that one of the plastic connectors coming from the power supply had a burn spot in it, right on the edge of the connector. The metal tang inserted into that slot was probably not conducting very well. I shaped it a bit and reinserted it- voila!- the four lights in the back board came back to life. That was the green wire, I think one of the "common" wires from the power supply.
Still can't get LED score display to come to life. I checked the logic board (I think that's the name of the circuit board on the back board) and I don't see anything out of line. It has been repaired previously in one place I think but I show continuity through that spot. What I did notice- I think- is that the red wires from the power supply had different voltage coming out of them. The middle red wire was lower voltage than the other two. I am referring to the red power wires coming out of the power supply unit.
I don't know if the display works off of its own power supply wire. Every light is working in the play field and the back board; in fact the only thing not working on this machine now that I can see is the score display.
I would replace the display module but this was working pretty fine about six months ago when I last used the machine. Seems like if the display was the issue it would lose lines slowly or intermittently and not go out all together at once. I have no display lights at all.
Does this help anyone identify the problem? I am at your mercy as I've actually exceeded my abilities altogether while learning even this much about this machine.