Yup. Looks like a circlip and threaded mount for the platform of a tripod where it screws into the mount of the camera. Now which model and manufacturer I could not say but judging by the weathered and worn appearance I'd say it was circa 1965. Good question to get people to post a comment... and not a bad photo either. Good detail - although I'm surprised at how little DOF you get at f/8 here. Do you have some adapter or extender installed as well?
