Vintage 1930’s Rhaco Clamp-on Series VII Filter Holder, Made In Germany.
Vintage RHACO brand series VII clamp-on filter holder for lens barrels from around 49mm to 54mm OD, made in Germany in the 1930s, Clamps on the front of your lens, no threads needed. Designed to be used with the thin glass Series VII filters only, those mounted in metal rings would be too thick.
Please check the photos closely for the exact measurements, they are part of description.