From what I know about the Elmar 50f3.5, the M-mount is the same formula as the late red Scale Elmar in screw-mount. It was introduced with the M3 in 1954 as a low cost alternative and about 13000 were made in all. It is a simple 4 element design but it is also an old design that was massaged over a period of almost 40 years and Leica improved upon it with new coatings and higher index glass. The 50/2,8 came later and is basically a f3.5 that was "opened" up a bit to 2.8. I suspect that Leitz used some of their newer glass in the 2.8 - but I never really noticed a huge improvement.
The 3.5 I find a better lens for close up and medium apertures. The 2,8 is by no means a bad lens, but I find that it lacks the "bite" of the 3.5. I have both the 3.5 and the 2,8 version I and the later 50/2,8 II. The later is better, but that is to be expected, another 35 years of knowledge accumulated, but like many of the newer lenses, it was designed for color, whilst the older ones were made for black/white. Contrast was deemed more important than absolute resolution. There is a certain satisfaction in using these old lenses. For one you appreciate how good the old "masters" were, f3.5 and 40 asa film for street shooting!
Many years ago I had the 28f5.6 Summaron. A small compact wide for the screw mount Leicas. It has impressive resolution, about as good as you will ever need, but the contrast is flat as a pancake! If you print with a normal filter in the enlarger, it does look a bit lifeless and bland, but by punching up the contrast you realize that it is tack sharp. The effect is very much the same with the 50f3.5. So it is a bit slow, a "fair wether lens", but collapsed on a M2 or M3 it is truly a pocketable camera/lens combo.
