With the lens off the camera how far will the retaining ring screw ? The problem is the shutter predates the modern standard set by Compur in the late 1920s. I have some older Compurs with flanges or retaining rings and just checked, the issue is the threads are coarser (less per cm) than the newer Compurs but the same diameter, so either way they'll bind almost as soon as you try to screw them on..

New Compur/Copal/Prontor #0 retaing rings are sold by Heavystar on Ebay at a reasonable price, however getting hold of the older type is near impossible. I'd go and buy scrap folders for the parts, I saw boxes full yesterday some as low as a 1 if broken.

Try junk shops, Charity shops etc, place a Wanted advert here. I'm short of these myself so have none to spare unfortunately.