Cap Rear Lens

Cap Rear Lens
How do I remove the Rear Lens Cap from a Canon lens?

Someone has donated a 135mm f2.8 Vivitar short telephoto lens to a charity shop that I have been helping at. The lens has a canon rear lens cap fitted so I assume it is a canon mount.
I tried to remove the cap to check the rear elements for fungus etc but it does not budge. I have tried a reasonable degree of force both anti clockwise and clockwise. I have also tried pushing and pulling for a bayonet type fitting. There are no obvious lugs or buttons and the lens mount looks like it has three bayonet slots that engage with a clockwise movement.
Can anyone help? I don’t want to damage the lens and my reputation as the shops photographic equipment specialist is at stake.

You most likely have an older Canon FD mount lens (discontinued and replaced by the current EF series). To remove the rear lenscam, rotate the chrome ring at the base of the lens. When it rotates, it will unlock the cap. To replace the rear cap or to mount the lens on an older Canon Camera, twist the chrome ring in the opposite direction.

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