My grandfather gave me a Schneider-Kreuznach Tele-Xenon 300/5.5 lens in excellent condition. However, it has a M40x1 thread.Are there any adapters to attach this thing to M42 or any bayonet mount? I can't find any on the internet, looks like M40 is an extremely rare thread.If I decide to order a custom adapter ring, does anyone know the flange focal distance of Praktiflex cameras? Sadly, the camera for which the lens was intended was stolen long ago.