I wonder if a 39mm to whatever step ring wouldn't screw right on there? Assuming that yours has a 39mm thread that is. Not at all sure if the thread pitch would be the same.
I recently found myself w/ two Federal enlargers, five lenses w/ different size threads, no baseboards that would fit any of the lenses, and no retaining rings for anything. So like the old saying, necessity is (often) the mother of invention.
The base boards were made from a variety of materials using different techniques (flat thin board, cut down and glued together plastic jar lids, and in one case, cardboard and thick mat board that I applied numerous thick coats of acrylic paint to strengthen). The lens retainers started out as flat pieces of plastic cut from thick jar lids, and I cut a hole in them that was a little smaller than the lens thread openings. After continuing to file and cut I got them to self thread on the lenses, and twisted them down until they were really tight. Worked perfectly. The lenses are true, and the prints are sharp.
