I agree with HowardR with: To every pin there should be a paper schematics.
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