Got an alligator jumper? clip at the power side, hit the coil lug first with the other end to see if it fires then walk it back through the daisy chain...

Later I'll need to look at the schematics, but should there not be a set of closed contacts
on the score motor's cam to keep the (SCR) Sequence Complete Relay energized until the
bank has reset? If manually pushing in the SCR, does it remain energized until the bank
resets? If yes, then there's probably another set of contacts in the R1 - R6 switches that's
not making. Bedtime now, but if still needed, I'll get to the schematics tomorrow (well later day)..

Okay, so the SCR does not hold itself when manually pushed in. There's a set of Score Motor contacts
(4C) in that circuitry, but that same set is in the BCR (black) circuitry, so that's not the issue.
There's a set of contacts in the U (SCR) itself that needs to be making for U to remain energized. It's
usually the very inside set. It should hold itself even if the R1 - R6 isn't making..
2nd, when the R1 - R6 switches are made, does the SCR energize for a split second? If not, with the
R1 - R6 switches made, you can jump R1 switch to the SCR blue-black (I believe) terminal (not the one
with the black wire) and that should energize SCR. If it doesn't, there's a set of switches in the Q (Series)
Relay (Red-Green) wire that need to be making. If Q seems to be fine, then you'll want to check continuity
on the R1 - R6 when they're made. Let us know what you find..
I can only check in periodically, so maybe/hopefully you'll get additional help for quicker response..

The sequence completed relay (U) is a special coil, A-17564. Check and make sure that hasn’t been replaced with the wrong coil. There should also be a 47 microfarad capacitor, a diode, a 1N4004 diode, and a 6 ohm 2 watt resistor in the on/hold path to this coil. If any of the connections have broken or one of the components has fried, that would explain why it wouldn’t stay held when you manually activated it.

I like to use a test light. You could connect it between the black wire on the coil and blue+black wire that goes to the resistor and see if it lights when you complete the sequence or manually activate the relay. You can make a test light out of two 12 volt auto light bulbs wired in series with some wire and alligator clips. Alternatively, you could use an AC voltmeter. Note that if you measure voltage at the coil, it is DC due to the diode.

Dave

Thanks autouncorrect! Assuming that was supposed to be "connect" lol

Right you are. Sorry.