Filters are mounted using a “filter adapter”. These were aluminum rings with a diameter slightly larger than the lens barrel. Various sizes were commonly available and we selected one that just slipped over the rim of the lens. The rim of the adapter was fluted. We rolled it on a table, reforming it to fit snugly. The other end accepted a filter with a smooth rim. A threaded retainer ring held the filter firmly in place. The retainer ring was threaded inside and out. The inside thread accepted a lens hood. These filter adapters were sized to accept specific diameter filters and lens hoods. They were labeled Series IV, Series V, Series VI, etc. We could get step-up and step-down attachments. Since we had a gadget bag filled with filters and several cameras, got the larger filter series and used the step-down ring. I have seen these adapters at photo swap meets.
