If you want to turn your M2 into a pocket camera, and you are looking only for a 50 lens, then the (two) Elmar(s), the coll. Summicron and the CV 50 f/2.5 are your obvious choices. There are also some collapsible Russians in LTM, the Summitar, and the older Elmar. Oh, and the collapsible Heliar - but if I am not wrong that last one is actually not so small despite being collapsible (never seen/handled one though, so I could be wrong). Of all these I 've tried for a while with the collapsible Summicron, the others were simply too slow for the kind of (portable) use I need (lots of interior shots, night photography), even more so as I don't like pushing beyond ISO1600. The collapsible Summicron did reasonably well in all that (you have to be careful with flare) although I suspect it will not compete with the modern Elmar (or your Planar for that matter) for sharpness and tonal nuancy wide open. But perhaps, like me, you wont care for all that if the circumstances you want to use the lens already degrade image quality anyway (slow shutter speeds, handheld fast shooting etc).
The camera with the collapsible Summicron just about fits a large coat pocket. Unfortunately M cameras are quite heavy and that sort of defeats the purpose of turning them into pocket cameras. I should know, I am currently trying the same ploy with another lens and the seams in my pockets are seriously disagreeing with the weight of the M.
EDIT: Oops, lots have been posted as I was composing. Nice photos from the Elmar.