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I bought this Vivitar lens cheap and I'm having trouble identifying the mount. I believe it's a Version 4 Serial number is 22128668
It is likely a Canon FL mount. If you are looking for an adapter, it is compatible with the Canon FD mount using stop-down metering. See Evolution of the Canon FD Mount.
If you'd like, you can measure the mount diameter to compare with a list of lens mounts (alphabetical, by-register, Wikipedia).
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