on page-10 (ori-9) the ipdb-manual "Dealers Choice" shows such a rare Unit - it is needed when the pin has a 5 cent coin chute - but the operator wants 10 cents for one play.
Confusing - Williams used the word "Alternator" also for a "Switch mounted on a Stepper" - I know it sitting SOMETIMES on the Number-Match-Unit.
My JPG-1 is from a page in a Williams-Catalog: http://www.planetarypinball.com/mm5/merchant.mvc?Screen=BOOK :
Z is the bakelite plate where the Number-Match is created. A is "place of the action". So B, C, D etc. - the coil pulls, armature is moved, a tooth hooks in, the axis with a 10-teeth-little-gear is turned one step (F hinders: turning backwards), G/H: a wiper on the printed circuit is stepped.
SOMETIMES there is also mounted: "I" (next to E) another gear with just ONE big tooth --- or there is mounted "J" another gear with 8 teeth -> the "follower-K" is moved upwards -> downwards -> upwards etc. -> the "Alternator-Switch L" is closed -> opened -> closed etc.
My JPG-2 shows such a stepper NOT having the Alternator-Switch. JPG-3 shows the No-Match-Unit in my "Space Mission" (looking at the unit from the other side). In my Unit the "K" rides NOT in the middle of "J" - it sits with a offset BUT it works -> K is moved -> Switchblade L is moved and closes to Switchblade M.
So the "Alternator-Switch L" lets a "Change-Relay" pull-in -> let go -> pull-in etc. And the Change-Relay let here: http://www.ipdb.org/showpic.pl?id=2253&picno=8473 the "Special when lit" move from left to right to left etc.
We should talk about Alternator-UNIT (Coins) and Alternator-SWITCH. Greetings Rolf