what is this relay called?

You sure you have a 72? Does / did it have seat belt interlocks? Different wiring harness?

On anything earlier with OUT seat belt "stuff" there are no relays of any kind in the start / cranking circuit except for the start relay out under the hood.

I'm wondering if you are seeing the horn/ "key in buzzer." Post a photo, or draw out the pattern of the connector pins and wire colors.

However, that should have nothing to do with cranking the engine.