Based on the mount description and the community answer, it is most likely a Canon FL mount. Canon FL lenses are closely related to Canon FD mount lenses, and they can be used on FD-mount film cameras with stop-down metering rather than full open-aperture coupling.

If you want to confirm the identification, measuring the mount diameter and comparing it with a reliable lens-mount reference chart is a good next step. That can help rule out similar-looking breech-lock style mounts.

If you’re shopping for adapters, look for Canon FL/FD-compatible options depending on the camera body you want to use it on. Just keep in mind that FL lenses generally won’t provide the same automation or metering behavior as native modern lenses.