Vintage temp sending units

[Lynn]

Three senders pictured.
1. AC 275 M. Pin connector with correct terminal end. I believe this is correct for 69 Camaro with idiot light. I also have been told it comes on at 275 degrees f.
2. AC 82B. Spade terminal. No idea. Guessing a sender for a light, but don’t know.
3. AC 12V (which I assume means 12 volt). I removed this from a 72 Chevy truck, C50 with a temperature gauge, so would ASSUME it is a sender for a gauge. Truck is inop, so I don’t know if it was also equipped with a light, but the other head had a pipe plug where the sender would go.

Three questions:
1. Am I correct on number 1?
2. I have also read that all the gauge senders used a threaded post connector. Anyone shed any light on this?
3. And can anyone confirm what numbers 2 and 3 are for and the significance of the numbers?
Thanks in advance.