Mystery dash light identification

[92470]

This weekend I was replacing a couple burnt out dash lights in my 71' Cheyenne. When I pulled out one that was burnt out, I noticed that it wasn't like all the others that are just the black twist in's.

This one had a male prong factory attached to the back of it, but there wasn't a lead running into it. My dash has all guages by the way, but no tach. Everything seems to work, so what is the purpose of this prong, and is something supposed to be attached to it? It is located in the middle left of the dash. Any advice is appreciated. Thanks!