Manfrotto original spare part, guide pin for quick plate that prevents the binoculars from rotating.
The pins fit the following quick plates:
384PL-14 200PL-14 500PLONG 501PL 504PLONG 509PLONG 200PL-38 MKBFRLA-BH MKBFRLA-BHUS MKBFRTC4GTA-BH MKBFRTC4GTA-BHUS MKBFRLA-3W