What sensor is this?
 

On the top of the bell housing there are the reference and speed sensors. Below them, on the same bracket there is a third sensor. What is it for?

Top dead center sensor. Only used for diagnostic.

Completely vestigial. I doubt even the dealer has the machine that used to hook to that sensor in the old days. You might contact Dan over at Ideola's Garage and see if he is interested in buying your sensor. Also a 931 part.