[m22mike]

The single pin or spade brass AC units are not senders, they are a simple NO switch. The # stamped on the is probaly the trip point where the switch closes and puts the ground on the idiot light. I would guess the internal contact is a bi metallic design.
The threaded post units are varistors, and send a variable resistance to a gage.
If you have VOM play with them. If you want to check the switch, heat the base in light oil.

Mike