It is one of Leitz's pre-Summilux designs (based on a Schneider Xenar?). They have soft coatings that are easily scratched and examples with clean glass can be hard to find. Filter size is an odd thread, and they are prone to flare when wide open. However, I love mine and it is very sharp when stopped down a few stops. The lens is heavy and has a solid feel to it. Mine was the first 50mm Leica lens I bought. I later sold it after acquiring more modern glass, immediately regretted the sale, and eventually replaced it with another. The first one was in better shape. It gives images a very distinctive, medium-to-low contrast 1950's look.
Ben
[Edit: should read Xenon 50/1.5, not Xenar]