Help ID this Napa Echlin...thing.

My first thought was like Dan's that it is part of an after-market electric choke. I had one from J.C. Whitney on my Dart slant that looked similar, though I recall only 2 wires. The big hole is so it fits on an exhaust stud. It had some type of thermal material that changed resistance with temperature to control the electric heater in the choke, similar to the later Mopar design (which was smaller).

Could also be an electric fan relay, as many suggest. 4 wires would be right - 2 for the coil and 2 for the contacts.

Final guess, there was a small box called a "load ..." something on F or J bodies that had to do with the alternator. Search for a post on that in the last few months. Dan solved that one. That part is in the 80's wiring diagrams as an optional part, as I recall. I think it had more than 2 wires, so might fit what you have.