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