We do not need to look at the Game-Over-Interlock-Relay. The Score-Motor starts turning AFTER You hit the Start-Button.
Warning / Danger ! 110 VAC / 50 VAC / 6 VAC may harm You -> WHENEVER You "just look at" / "work at" the pin - First: unplug / pull the 110 VAC Main-Power-Cord (NOT: just switching the Main-Power-Switch). Plug-in the Main-Power-Cord ONLY FOR TESTING.
I want You to understand what / why "we are looking at".
To have a look at the schema: I must make a copy onto my computer: "Right-hand-side-Mouse-click" in ipdb.org -> "Save target" -> Your computer.
The connection "50 Volt "POWER"": transformer (C-2) -> 10 ampere fuse -> wire BLACK -> to the right -> -> one-side-of-Score-Motor (E-9).
The "returning connection": transformer (B-2) -> wire YELLOW -> -> a "Branch-off" at B-4 / B-5, but keep on straight / horizontal -> at B-9 upwards towards the Score-Motor through one ore more "Switches at" Outhole-Relay / Bonus-Score-Relay / Lane-500-Relay / Top-Hole-Relay. All these switches are O.K. as the Score-Motor NOT starts turning when You turn on the Main-Power-Switch.
The error must be on switches (to the left): Coin-Relay / 2nd-Coin-Chute-RELAY / 3rd-Coin-Chute-RELAY / Reset-Relay. The error can NOT be (a bit to the left) "Score-Motor-Switch-1-D (as the Score-Motor NOT starts turning when You turn on the Main-Power-Switch).
futurepinhead, please look in the boomerang for 2nd-Coin-Chute-RELAY / 3rd-Coin-Chute-RELAY. When You start a game: Does one or both start pulling ? Please report.
Maybe the fault is on the Coin-Relay (switch) - I assume (more likely): The Reset-Relay starts pulling (thats O.K.) - BUT it never lets go (the fault). Why does the Reset-Relay "never let go" ?
Schema B-11 / C-11 / D-11 / E-11: I see Reset-Relay / Game-Over-Relay-Latch / No-1-Score-Reset-Relay / No-2-Score-Reset-Relay.
futurepinhead, please look for Reset-Relay / No-1-Score-Reset-Relay / No-2-Score-Reset-Relay - after You start a game: Does the Reset-Relay keeps-on pulling and the No-1- and No-2- relays are pulsing (on -> -> off -> on -> off ...) ? I assume "Yes" - please report.
On C-11 I see switches: "16 Drum Units Zero" / Bonus-Zero-RELAY / Score-Motor-Switch-8-E. If one (ore more) is "faulty-closed-(forever)": Your "Boomerang-Problem". You must check all these switches - lets start with "Bonus-Zero-Relay": Look schema-F-4: the Coil of Bonus-Zero-Relay. This Relay can pull (it must pull) when "switch on Game-Over-Relay" AND "switch Bonus Unit Zero" (both) are closed - that means: As long as there are some bonus on the Bonus-Counter: Bonus-Zero-Relay is pulling - when (truely) there are no bonus on the Bonus-Counter: Bonus-Zero-Relay does NOT pull.
To end this post - futurepinhead, please look in the Boomerang for the Bonus-Zero-RELAY -> when the Score-Motor is (faulty) turning: is this relay PULLING ? (it should not).
Have a look at the switch mounted on Bonus-Zero-Relay, wire "YELLOW" and wire "RED-YELLOW" are soldered-on. (When the relay is not pulling) - Is this switch "faulty-closed" ? Do the solder-lugs have "faulty-connection" (blades bent / a solder splash in between) ? Please report. Greetings Rolf
P.S.: We might have to look at several places - eliminating "this is NOT the fault" -> "this is NOT the fault" ...