Found it, according to the williams parts manual ( http://www.planetarypinball.com/reference/partsmanuals/WMS_Parts_1977-1978/index.html#/32/ ) it has a coil A-22-550, your pic on first page matches this so I going with the 'Ball release coil' on the schematic. This is directly fired by the outhole relay.
If you didnt do it when you had the apron up (and even if you did) I think the next logical step will be to test that coil, maybe re-solder its wires and make sure its moving freely.