Anyone know what this is?

Looks like you sort of got a answer to your question on another forum.
The seat belt interlock system was not used in 1976.
What you have is a seat belt buzzer which also controls the seat belt warning light.
That particular design was only used in 76 for the Abodies.
It was also used on some Mopars from the late seventies.
Over the years the buzzer was used, the color changed several times but other than that they were identical.
The number on the buzzer is a factory id number not the part number.