The FD lens mount is a breech-lock mount, which is a variation of the common triple-flanged bayonet attachment. The advantage of the breech-lock over the bayonet is that neither the contact surfaces between the body and lens, nor the signalling mechanisms, rotate against each other when the lens is mounted. This prevents any mechanical wear, which could conceivably reduce the very precise lens-to-film distance or introduce communication errors between lens and body.
Lens body overall is ok
Aperture speeds ring seems to be moving fine
Lens has sign of fungus, dust & marks
Lens focusing ring seems to be stuck and wont move
Due to the issue, this item being sold as is - No Return