What is this thing???

With the wire connector disconnected from under the seat, the system no longer works and the car will start normally.
My Dad sold new cars from 1967 to 1994. In 1974 he sold Chevrolets. They had a similar system and since almost nobody wore seat belts then, this was an annoying thing to deal with. He would get a car from the lot as his personal car and reach under the seat to disconnect the wire plug. Nothing permanent, just a temporary thing so the car would start like before.