Ken, thanks for the info.
I have a Williams black Coinco door. It is in a Moon Patrol cabinet.
I have learned a few more things by checking around the internet. Although, there is very little information on lockout coils.
Mine are missing the spring that activates the lockout. That's why I couldn't figure out how they work. I checked Happ Controls and looked at their picture and saw the spring.
When power is off, the spring allows the metal tab to swing into the coin mech opening and it sends all coins to the coin return.
When the power is on, the coil creates a magnetic field and pulls the tab away from the coin mech and allows it to process coins normally.
Kind of a neat feature. When the machine is off, no one can lose a quarter

Now I just have to figure out what voltage it is rated for. The only identifying mark on it (besides "May 1982") is 904218 (presumably the part number). This doesn't match any of the part numbers at Happs. Anyone know about this? Anyone have a Moon Patrol Manual that has this information?