Hi. To OP (or anyone else with any ideas) - I have an opemus 5 and just bought a rodenstock rodagon 2.8 50mm for it as well. The M39 thread fits fine, but the actual case of the lens hits the rounded recessed part of the standard opemus lens board when you try to attach it in the inverted position necessary to get focus in 50mm and so it seemingly cannot be mounted.
Did you get this to work? I am struggling to work out what to do short of getting a custom lens board made up, if that's even feasible.
Really annoyed actually cause it seems like a great lens!