Hi uj
I refer to: https://pinside.com/pinball/forum/topic/what-does-an-alternator-unit-do#post-3203672 the close-up picture of (here) post-11.
I have never seen this thing moving - I do not know if it does alternate helping on "TWO coins for ONE game ???" I do not feel like "investigating on that" --- do I want do damage my running Space Mission ? ...
You are missing "the part 'D' and the spring and the holding axis": https://pinside.com/pinball/forum/topic/what-does-an-alternator-unit-do#post-3203050 --- look at Your functioning "Aztec-Unit" - see how 'D' is pressed against 'E' / 'I' 'J', the nylon-gear, so when 'C' is moved then 'D' hooks-in under the next tooth on 'E/I/J'. Want to take an old can of beer/coke and an old scissor and make an "homemade new 'D' - an leaf blade standing upright - pressing slightly against 'E' " ?
I live in Europe - I never had a look in "pinside - buy and sell subforum" - want to try to buy an spare "Williams No-Match-Unit (with or without the alternator stuff)" - as You only need 'C' and 'D' ???
I wrote about NEW Relais: https://pinside.com/pinball/forum/topic/chicago-coin-sound-stage-a-thru-f-target-issue/page/2#post-3188493 --- in Europe I would try to buy a new FINDER-Stepper, YOU in the USA - want to investigate on: http://www.ia.omron.com/products/family/965/specification.html --- is this an "on - off - on - off etc." stepper. If so, then You could use this for the activation / de-activation of "Change-Relay" (of course, You will not have the No-Match-functionality).
I assume FINDER does not make (no more) AC-Steppers, I would investigate "WHAT is DC-sensitive ?" and maybe buy from here: http://www.tracepartsonline.net/(S(dk5fhlm1hmvcj20l5mivhozn))/PartDetails.aspx?Class=FINDER&clsid=/F_FINDER/FINDER.010/FINDER.010.010/&ManID=FINDER&PartFamilyID=10-30012009-084100&PartID=10-30012009-084100&SrchRsltId=5&SrchRsltType=0 an 24 Volt DC Stepper - as Space Mission HAS an bridge-rectifier to produce 24 Volt DC for Bumpers ...
(((You may ask the OMROM-people about 24 Volt DC ?)))
Back to "making Your own stepper": Take an old Score-Drum-Unit with an plastic-wheel mounted (for the player to see the points) --- in position-0, pos-2, -4, -6, -8 You glue on a stripe of metal left to right onto the plastic wheel, You mount two leaf switches riding on the plastic wheel --- in pos-0, -2, -4, -6, -8 BOTH leaf switches ride on the glued-on metal: current can flow --- AND You have made Your own "Alternating Stepper".
Greetings Rolf