On a Williams EM, the index cam and the impulse cam are always located on the opposite ends of the score motor assembly. The impulse cam will have a bunch (maybe 10 or 12) of "teeth" that push up the switch bank follower while the index cam will be smooth except for maybe one notch. The impulse cam is almost always on the far right side as you look straight in at the end surfaces of the plastic discs (cams)

I forgot that the Doodle Bug schematic is online at ipdb.

In the link below, the impulse cam is the rearmost one.
https://www.ipdb.org/showpic.pl?id=703&picno=41663&zoom=1