Thick washer between crank inner face and freewheel body of freewheeling crank.
The crank would otherwise run out of threads on the freewheel and wouldn't sit flush otherwise, weakening it and causing the threads between crank and freewheel to gall from moisture intrusion. (Crank is aluminum, and freewheel is steel. Of course, I used liberal amounts of anti-seize on the threads.)