Well, to close this thread I started, I successfully installed my original stock switch with an 85 cent jam nut today. My case shows no signs of ever being threaded. In fact, the inside of the case has a machined flat area where it was very clear a jam nut had been earlier in the life of the car. I measured the stack up tolerances to ensure the switch probe was not binding in reverse. FYI, a stock switch moves 6 mm full stroke with mine reaching continuity at 3mm. So you have 3 mm of float to play with. Put a couple small washers on the outside of the case to position the switch correctly on the inside and tighten the jam nut. FYI the stock switch is a 10mm x 1.0 thread. While I was in there, I took the liberty to lube the shift mechanism rod and ball with white lithium grease. Put it back together and reconnected the wires. FYI the green wire (white and green) is the switched hot when the lights are on. No backup lights unless the lights are in park or headlight position. Taking the cover off if I ever have to remove anything is painless so no issues with the nut being on the inside. cheers and thanks for the input.
Now to replace all the rear bulbs with new stuff. Going to clean all the grounds that support the rear lighting as well as sockets, etc.