It is not the best lens on the NEX system but likely the thinnest on a tight budget. It is a Russian made Industar 69 28mm f/2/8 pancake in M39 mount
More in this flickr set for Industar 69 28mm f/2.8
On the lens, there is a extra ring and I happen to need to take the extra ring out so that I can use my NEX without issue -- I still don't quite know what the extra ring in the rear is used for. It is likely an adapter between M39 to LTM/L39. On the L39 to NEX adapter, I happen to need to re-adjust the three mounting screws in order to align the lens right with focusing scale.
I have to use it more and report back on its performance. So far what attract me most about this lens is the pancake thinness and its small size of a 28mm prime. I am not sure if a 25.5mm filter size or cap will fit, I will report back when I have found the cap and hood size that fit this pancake.
There are other users complaining on infinity focus and requirement of modification. Contrary to what I have heard from others, I can use this lens without any modification except removal of an extra ring on the rear of the lens.