Reverse switch?

According to the 69 shop manual, should be the 3 pin on the transmission. That's not to say some "mid model year" changes might have transpired.

You clearly have a shifter mounted reverse switch. The older ones had a single pin switch on the transmission for NS, and a separate switch as shown on your shifter for reverse lights only.

If it's not wired up, someone obviously bypassed it. You should have a starter relay with 2 "push on" connectors, and one of them should go down the NSS pin on the transmission