It's ebayese, I think. L39 = Leica 39. M39 = metric 39, that is 39mm x 1mm pitch. Both are somewhat inexact, as the major diameter of the threads is indeed 39mm, but the pitch is 26 threads per inch. Neither 'M' nor 'L' designations distinguish adequately between the 39mm lenses for rangefinders and the 39mm lenses for the FSU SLR cameras.