I picked a lovely condition Sigma f8 600mm mirror lens for next to nothing and took a chance on the mount. It is neither a Nikon F nor Canon FD (pre autofocus) lens mount. The inner diameter (chrome edge to edge) is approximately 39mm. Can anyone tell from the pictures what it might be?
Yes, it's Oly. See the two squeeze tabs (located at 10 and 4 o'clock in the top photo) that are used to mount/unmount Oly lenses? That's the dead giveaway. The Sigma 600/8 mirror can be a great optic. I bought a copy back in 1984 in Canon FD mount and it was an exceptionally sharp lens.
Do know that the OM4s, at least many of them, have an unpleasant habit of discharging their metering cells rapidly. Other Oly owners may be able to comment more fully.
I had a choice: sell the lens or get a camera for it. Bought a OM-10 for $110, looks in good condition and has a return if unhappy guarantee. Looking forward to the first roll ever with a mirror lens.
Edit: PS the lens was $68