I'm pretty familiar with that machine. Maybe I can help if you have questions.

An alternator is an electronic or mechanical means to facilitate 2 coins for 1 play.

On some older coin-op equipment the first of two coins land on a mechanical seesaw. The seesaw rotates/flips, directing the coin through an alternate path away from the credit switch and lands in the coin box.
The second coin resets the seesaw which directs the coin to strike the credit switch for a vend before dropping in the box.

On the Space Mission schematic, the first coin hits the credit switch and momentarily turns on a coil. It would cause a shaft to turn, closing an alternator switch. The second coin hits the coin switch and the coil pulls in again, and a vend is made through the alternator switch.

I don't recall seeing one in a Space Mission.
Strange that it is not listed as an option on the schematic.