The Alpex and Rapex #1 shutter had about 30mm shutter mounting threads. So did the #0 Compur and Ilex #0 Universal, Acme, and General. As Paul noted, some retaining rings were intended for larger holes. They still work with thread diameter holes. The old standard 4" square Lens boards are fairly easy to build up from 1/8" plywood, or mill down from 1/4 plywood. For temporary use, they can be improvised from mat board, foam core, or whatever else is available.