here are the notes from my site:
35 Summicron F2 Type 1
Introduced 1958. Many users believe the Type 1 is the best non-aspherical version ever made. There are two versions of Type 1; one is the "M3" type which has "eyes" attached to allow the lens to be used on the M3's finder which doesnt cover the 35mm view. The other version is called the "M2" type which is a regular M mount lens that can mount to any M-mount camera. The M3 type will work on other M cameras, but it will bring up 50mm framelines. The eyes can NOT be removed, else focusing errors will occur. Both types came in chrome and in black. These lenses had 8 elements and provided excellent resolution, but contrast was middle of the road. Many users love this version for classic black and white photography. About 12,000 M2 types were made; 10,000 M3 types. Close focus .7 meter (M2) and .65 meter (M3). Most of the Type 1 lenses were made and marked in CANADA. Some were made and marked Wetzlar.
35 Summicron F2 Type 2
Introduced 1969. Elements reduced from 8 to 6 in this version to improve contrast. Series 7 filter size. Contrast is improved over Type 1, but resolution is not quite as good. Some had an infinity lock, others did not. Lens came only in black. Aperture adjusted with a tab. Close Focus .7 meters.
35 Summicron F2 Type 3
Introduced about 1971/2. Optics changed slighty, infinity lock gone. Performance is basically identical to previous version. Some early ones may have Series 7 filter, but then all E39 filter. Lens came in black only after 26xxxxx. Close Focus .7 meters.
35 Summicron F2 Type 4
Introduced 1979. Known as the "Bokeh King," this lens helped start the bokeh craze of 10+ years ago. Optics changed to 7 elements. Two versions - one with a convex focus tab, the other a concave tab ( see images below ). Originally made in black only; about 1994 some chrome versions were produced. Performance is improved over previous version, but coma and astigmatism remain in outer zones ( contributing to the nice bokeh ). E39 filter. Close Focus .7 meters.
35 Summicron F2 ASPH
Introduced 1996. 7 elements including asphere surfaces. Significant improvements in contrast, resolution, vignetting. Technically, one of the finest 35mm f2.0 lenses anyone has made. Chrome, black and titanium versions available. Close Focus .7 meters.