Early shifters have a pin in the shifter's end ball,and a slot in the shift rod cup to keep the shifter itself from rotating. That's exactly what i have as you state Justinh.
Now what i think is going on is the pin is sliding out of the shift rod cup and allowing to rotate.
Now what's the fix? I'm working on it and i'll ultimately get it fixed, but i want do it myself, that's part of the reason why i got the car, it's a hobby.if I really have to take the car in for major work i will this doesn't seem to be major, i've done all the work on most of my other cars, adjusting valves, tune ups, brakes,oil changes,elctrical, and so forth.
I'm still not clear, on aftermarket shifters as you describe it. is this the fix.
I was told that there's a bushing on the shaft that sometimes were's out, and that may need to be replaced ?
Thanks, Art