Hi
I agree with HowardR with: To every pin there should be a paper schematics.
n1teowl -
please read the whole post - then decide which way You want to do the testing.
I wondered what other TWO-Player pins Williams made at the time - I used the advanced search in ipdb: http://www.ipdb.org/search.pl?mfgid=413&yr=1970-72&pl=2&sortby=date&searchtype=advanced - I looked through the list and: Strike Zone http://www.ipdb.org/machine.cgi?id=2404 is close in time and ipdb has the schematics --- I show a snippet (sqeezed a bit) -

I believe in Your pin functions the "1,2,3,4,5" - the Ball-Count-Unit steps-up one step. In a ONE-Player-Game there should be a second*** stepping on the Ball-Count-Unit: "1,2,3,6,7,9,11,12,13".
The problem in Your pin is an nice example of "lets use an SteveFury-Test-Light" - it is shown here: https://pinside.com/pinball/forum/topic/reading-gottlieb-schematics#post-2634425 .
Toggle-off the pin, unplug the main power cord (Safety Reasons) - then establish an permanent wiring with the Test-Light clipping-on at "9" and "14". Instead of "14 (anywhere on wire-black)" You may use "16" or "15 - the fuse - BUT use side wire-black on the fuseholder - so You have the fuse in the wiring". Plug-in, toggle-on and try an ONE-Player-Game - Does the Test-Light lights-up ?
IF (if, if) "No luck": Toggle-off, unplug the main power cord and establish permanent Test-Light-connection "7 to 14" and do test.
IF (if, if) "Still no luck": You have proven: The fault must be in "3,6,7" - somewhere in this wiring is the fault (well, one fault). Please write about Your findings.

IF (if,if) the Test-Light DID light-up - You have proven: The wiring and switches "3,6,7,9" is good - the fault must be in the wiring "9,11,12,13". Please write about Your findings.

I like to use a Test-Light --- but You can do tests "NO Test-Light used" - just using an Jumper-Wire and do permanent Jumpering: "9 to 5/13" - "7 to 5/13" --- big question: Does the Ball-Count-Unit step-up ? Please write about Your findings.

Please use http://mirror2.ipdb.org/files/2240/Williams_1971_Solids_N_Stripes_Instruction_Manual.pdf - on page-25 (ori-23) on the 2nd-Coin-Relay it is switch-D - my "7" is the middle blade, Gray-Blue --- my "9" is the bottom-most blade in this switch, Red-White. Therefore the Ball-Count-Unit-Step-Up-Coil has (as shown in my JPG): Red-White and Black".

second*** stepping --- actually this stepping is done before --- as SCM-1B closes earlier in time - before SCM-2B closes. Greetings Rolf

0Strike-Zone-Work-02 (resized).jpg

Hi n1teowl
the Test-Light does not light-up --- in Your video in post-5 it does light-up --- but lets say "we test the Test-Light": In the lower left corner of my JPG in post-4 we see "orange 1" - a Make-and-Brake-Switch on the Outhole-Relay --- on page-23 (ori-21) in ipdb-manual "Solids-N-Stripes" I locate the switch as the Switch-D - wire-Yellow on middle blade - wire-red-white on "blade to get-closed-contact when relay is pulling" - JUST as in the JPG. When You hook-up Your Test-Light "Solder-Lug-of-wire-red-white on switch-D on Outhole-Relay" to "my 14 or 15 or 16": The Test-Light will light-up as long as the Outhole-Relay is pulling - does it in Your pin ?
(((The written above test You may do - or You do not do)))

In the JPG here I made "huge, big, rosa-pink X" --- one or more place(s) the fault(s) is/are.

From the manual Solids N Stripes I take: "Outhole-Relay-Switch-D-wire-red-white" runs to "Ball-Index-Relay-Switch-B". From this switch-B then runs wire-red-orange to "Extra-Ball-Relay-Switch-D". From this Switch-D then runs WIRE-ORANGE-RED*** ---

What we see in an schematics is "abstract - drawn beautyful" --- in reality it is more like in the JPG here - see upper right corner: The wire from the Extraball-Relay runs to a Score-Motor-Switch - and from the Score-Motor-Switch hops a short wire over to the other cam / wheel - Switch.
So WIRE-ORANGE-RED*** coming from the Extraball-Relay MAY arrive at Score-Motor-Cam-1-Switch-B or it may arrive at Score-Motor-Cam-2-Switch-B (or any other cam - any other switch on a cam --- we do not have the schematics, YOU MUST look-up in Your pin) - a short jumpering wire of any color (maybe even a bare/naked wire ?) connects the two switches on the two cams - from one cam (probably cam-2-switch-B) runs a straight wire (hopefully***) to "Step-up-Coil on Ball-Count-Unit", from the other cam (probably cam-1-switch-B) runs a straight wire to "Switch-D on 2nd.-Coin-Relay".

You must locate the cams (cam-1 ? cam-2 ?) and the switches on the cams (B ?) - and the connecting wires - soldered-on ? contact-points on the switchblades clean ? switch (probably SCM-1B ?) closing when the running motor actuates the switch-stack ? Greetings Rolf

P.S. - hopefully***: Remember, schematics are drawn abstract - the wire may run to "Switch on 2nd. Coin-Relay" - and then "Switch on 2nd. Coin-Relay" is connected to "Step-up-Coil on Ball-Count-Unit".

0Strike-Zone-Work-03 (resized).jpg