I bought this SOM Berthiot 20mm f1.9 lens assuming it was a c-mount, but it is much smaller with an elongated screw mount. Can't figure out what type of mount this could be.
Here it is compared to a regular c-mount lens:
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The lens in the picture is either a Bolex 8mm/9.5mm or 16mm format motion film camera.
This is a French cine lens. Monsieur Berthiot workshop. Society Optical Mechanical is my translation of SOM.
It is definitely not for 16mm. Not for the D mount cameras, but it must be for 8mm or 9.5mm cine. Not for the Swiss Bolexes. It seems to be proprietary to Berthiot and unlikely to fit anything made by anyone else. 20mm would be ~ 115mm in 135 format or FF in recent parlance. So a portrait lens.
Might be convertible to Pentax Q but the registration distance might be an issue. DIY adapter, I an afraid.