One point relay? Score reel EOS switch?

I'm that that familiar with Bally reels but most score reels have their regular group of switches with a separate EOS switch. It can be easy to miss depending on it's placement. On some designs if that switch doesn't open the score reel locks on.

Another thing to look for, and again I'm not sure it applies to 60s Ballys are tension blades shorting a switch. I know you already checked the playfield switches but if it has tension blades between them they can sometimes be misadjusted to touch both switch blades. Easy to miss if you focused on contact gaps. I like to lift the playfield and look with a flashlight from underneath.

Sorry I told you to look where you already looked but those are the obvious places. For something weird to check in this situation I always check the playfield harness near the back of the playfield. I have had a few games with worn away cloth insulation if there is a sharp edge near where a harness bends. Obviously this can cause a short with varying results. My first Teacher's Pet would always spot me the "P". A recent Pro Pool lit multiple balls in play. Both drove me nuts for a bit.

The schematic will have clues. Do you have one? Post a copy here or even better at IPDB.

You might also have a switch that scores one point stuck closed. This sometimes happens if you replace the rubber rings.

Lamp socket or bare wiring making contact with one of your switches ?

Please post your schematic to IPDB. That will also help us help you.