Found an unconnected wire while under my 72 Cutlass 350 with 350 THM. It is black and has a round rubber plug/connector (about 1/2 inch diameter) with 2 slits for plug in. One horizontal and one vertical, but it looks like only one slit is wired. I found it along the drivers frame rail while connecting lines to the brake combination valve. I suspect it goes to the transmission controlled spark. Does that sound correct?

 

That's what its for.

 

I'd like to guess, for a car that had a 400 turbo transmission and not used on your car with the 350 trans. BUT THAT IS JUST A GUESS, someone will let us know for sure.