Helios-44m, black lens for SLR camera
58 mm, f/2, M42 mount, s/n 001956
Krasnogorsky Mechanical plant (KMZ), Krasnogorsk, USSR

Perfect condition. Lens is working properly. Glass does not have any fungus or scratches. Inside there are microspecks of microdust. Focus ring rotates properly and tightly. Aperture ring rotates properly. 

Camera "Zenit B" is not included and shown as an example of mount.

This lens was designed at Krasnogorsky Mechanical Plant. This is a representative of vast "Helios-44" family, that was based on German Carz Zeiss Jena Biotar 2/58. Firstly it has a name "BK" - Biotar from Krasnogorsk.
Optical scheme was calculated in 1951 at State Optical Institute. Lens was produced in 1958-1994 at three plants: KMZ, BelOMO, Jupiter with four possible mounts: M39, M42, Pentax-K mount, Start. This is one of the most popular lenses in the world.
This version has 8 rounded blades. This lens has a jumping diaphragm. Red letter "R" on the body must be put when taking picture on infra-red film. Lens has a switch "A-M" (A - for automatic and M - for manual aperture control).
When mounted with adapter on Canon, for example, its focal distance will be: 92,8mm (for APS-C), 75,4mm (for APS-H).

Characteristics:
Lens s/number - 001956
Number of lenses - 6 in four groups, Biotar
Angle of view - 40o
Number of aperture blades - 8 
Limit of aperturing - 1:2 (max), 1:16 (min)
Resolving power - (centre/edge) 38/19 lines/mm
Minimal distance of focusing - 0.5 m
Type of thread - M42 mount
Type of thread for the nozzles - 52 mm 
Dimensions (mm): 42*60*60
Focal distance - 58,60 mm.
Mass - 287 g 
One-layered anti-reflection coating