Helios-44, black lens for SLR camera
58 mm, f/2, M39 mount, s/n 7050911
Belorussian Optical-Mechanical Unity (BelOMO), USSR

Very good condition. Glass is clear and does not have fungus, there are cleaning microscratches on the front. Inside there are microspecks of dust. Aperture and focus rings rotate properly. Body has signs of age and use.

Camera "Zenit-3M" is not included and shown as an example of mount.

Note: it is a lens for SLR with mount M39x1/45.2, it will not work if set on rangefinder with M39x1/28.8 (LTM) mount. They have different flange focal distance.

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 (USSR). Lens was produced in 1958-94 at three plants: KMZ, BelOMO, Jupiter. This is one of the most popular lenses in the world.
This lens was default for SLR cameras "Zenit-3", "Zenit-3m", "Crystal" and optional for "Zenit" and "Zenit-C". This lens had either 8, or 13 aperture blades. Early lenses had unpainted aluminium body, later lenses were black.

Characteristics:
Lens s/number - 7050911
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) 35/14 lines/mm
Minimal distance of focusing - 0.5 m
Type of thread - M39х1/45.2
Type of thread for the nozzles - M49x0,75 mm
Dimensions (mm): 61*60*60
Focal distance - 58,60 mm.
Mass - 220 g
One-layered anti-reflection coating