I'm planning to mate a Petzval to a Betax #5 shutter, for which I need to machine an adapter ring. The Petz has a flange, so no problem affixing it to the adapter, but the Betax has female threads, somewhere in the vicinity of 2.93 inches in diameter. Does anyone know the thread specs for a #5 Betax--diameter and threads per inch (English or metric)?