Version I (9 elements in 6 groups) had a large rear extension that cut off too much light from reaching the light meter of the M6, M7, and MP (and of course could not be used with the M5 or CL without modification). Version II (8 elements in 6 groups) had less back projection and could be used with all Leicas; but the optical formula was worthy of improvement. Version III (also 8 elements in 6 groups) was optically improved, and is excellent; though it does intrude too much into the finder frame. Version IV (8 elements in 7 groups) is shorter, and intrudes less; at the same time, it is once again improved, to the point of being outstanding. Then there is a recent addition to the line, which if memory serves, has an aspherical surface--with all the benefits and drawbacks thereof.
Some people prefer the older version of practically any focal length! Older lenses are less well corrected for spherical abberation, as well as the other basic abberations. When the abberations that do exist are well balanced--as they are in old Leica lenses--they impart a certain pleasant quality, often described as "plasticity" or "roundedness" to the picture. The image may not be as sharp, or have as high a resolving power, as the newer designs, especially the aspherics; but not everyone is after that. Thus the older lenses hold their value and are sought after.
