68 Dart GT 10" Front & Rear brakes

The pin is for the self adjusting bar and cable, (not E brake cable), that in theory automatically turns the star wheel, and spreads the brake shoes when you brake while backing the car. When its together you can squeeze your thumb on the cable toward the shoe and see the action.
The cable (w spring) attaches to the bar, which slips on that pin. The eyelet end of the cable attaches on the top of backing plate.
It sounds more confusing than it looks. See pic.Screenshot_20201213-225710_Google.jpg