Genuine original Canon part. BRAND NEW, NEVER BEEN USED.
Canon parts # YF2-2101
It's for the listed Canon parts:
EF 300 mm F2.8L IS II USM
EF 400 mm F2.8L IS II USM
EF 500 mm F4L IS II USM
EF 600 mm F4L IS II USM
EF400mm F4 DO IS II USM
EF 200-400 mm F4L IS USM Extender 1.4x