The Zenitar-M 50mm f/1.7 was produced by KMZ (Красногорский механический завод) in the 1970s-1980s. It is quiet a chunck of glass really... There also exists a multicoated version (MC Zenitar-M 50mm f/1.7, which should not be confused with the MC Zenitar-M 50m f/1.9) and there are variations with Latin scripts. The Zenitar-ME1 50mm f/1.7 is a rather uncommon lens that has only two aperture blades.
The presented lens is listed by Princelle as K1694:
Aperture range: f/1.7-f/16
Minimum focussing distance: 0.45m
Aperture blades: 6
The lens also has an A-M switch, very usefull if you don't have a good M42>NEX adapter... Some images of this lens in combination with the NEX-5:
Picture quality of this lens is quite decent. Sharpness is OK, especially in the center; the bokeh is buttersmooth. There is some chromatic aberration wide open. I like this lens especially for B/W-photography. Some examples:
f-stops: 1.7 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16
You can find more examples in Boring Lens Tests FLT.1 ;-)
Some "real life" examples:
NEX-5 + Zenitar-M 50mm f/1.7 (ISO400, f/1.7, 1/2500s)
NEX-5 + Zenitar-M 50mm f/1.7 (ISO400, f/11, 1/250s)
NEX-5 + Zenitar-M 50mm f/1.7 (ISO400, f/1.7)
More examples can be found in the dedicated gallery on this website.
The ARG-file I use with exiftool: zenitarm50mmf17.txt
-XMP:Lens=Zenitar-M 50mm f/1.7 (M42)
This works for LR4 and NEX-5 RAW-files.