Canon YA2-3463-000 Rubber, Mount is a replacement component for the mount assembly of the Canon EF 500 mm 1:4.0L IS USM lens. The parts list identifies this exact code as the rubber mount, helping distinguish it from other rubber rings and seals used in the lens.
The listing names Canon as the parts manufacturer and confirms the full item code YA2-3463-000. The parts catalogue specifies one rubber mount for the lens. This is a dedicated service part for that assembly, rather than a general-purpose rubber ring.
Use this part when servicing the mount assembly identified in the corresponding Canon parts list.