this relay is for what??

The seatbelts had an electrical circuit which operated with that relay, you could not start the car without the belts hooked up. There was an easy way to disable the system, you could unhook the connectors to the harness under the seat.

I don't remember, I think the button was for an emergency start if the system failed. People raised hell about them, and they were eventually changed to the warning lamp/ buzzer you see today, with no engine interlock.

AS I said, if you think that's what it is, remove it. The engine should not crank. Jumper the two large yellow leads, and it should crank. Of course if you have an auto, you much have it in park/ neutral, and if you have a stick with the clutch safety switch, the clutch pedal must be depressed.

I wish to heck some of these guys with later shop manuals, 73/4/5, etc, would post them in the "download" thread. They have to be "out there."