Is this the image you are talking about?



Looks like I have a weird one-off configuration. That's the same hole, right down to the notch through the threads, but it has a different kind of overload protector, underneath a plug, and the kickdown solenoid located in a completely different spot.

I've got this,

screwing in to this hole,

topped off by an electrical connector.








I guess at this point there's nothing for it but to dive in and see what's in there.

Thanks again for all the help.