lens - What is this breech mount with three flanges and a single linking pin?

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

Answer

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).