Lock Knob Nut for Lens Hood ET - 155 Canon EF (RF) 400mm f/2.8L IS II / III USM
This is non-Canon alternative Lock Knob Nut for Lens Hood ET-155 Canon EF (RF) 400mm f/2.8L IS II / III USM. 'Great bit of kit - actually better than the original. Confirm it works on 400mm IS II and 200-400'.