It is easier to draw the schematic with logical functions, than trying to keep all the connections in one unit together. In later Williams games, there was a section in the manual, that described every relay and unit by their switches. But in 1962 they only included the schematic.
Score drum unit switches were usually labeled in 60's schematics as "0-9 EOS SW", "0-9 ZERO SW" and "9 D.U". Try finding the 10 point relay coil in schematic and see what energizes it. The relay of course will operate when any 10 point switch in playfield closes, but also when 1 point is scored and the 0-9 drum is at 9.
It takes a while to get familiar with EM operation and schematics, but you will get there. Good luck!