FED
foldable (collapsible) silver lens
#238186
50 mm, 1:3.5, M39 (LTM) mount lens, "FED" plant, Kharkiv, USSR

Very good condition. Lens is clear and clean, without fungus or scratches. Inside there is dust. Aperture ring rotates very-very tightly, focus ring rotates properly. Body has small signs of age and use. 

Camera "Zorki-4" is not included and shown as an example of mount.

This lens is a copy of German "Leitz Elmar" and was produced in 1934-55 by "FED" plant in Kharkiv (before the WWII), in Berdsk (during WWII), at KMZ in Krasnogorsk (after the WWII). Lens had foldable (collapsible) version for rangefinder camera and unfoldable versions for SLR camera "Sport".
After the WWII got name "Industar-10". In 1952-55 was produced with one-layered antireflection coating. Flange focal distance is approx. 28.8mm, so lenses needed additional adjustment.
On its base later "Industar-22" and "Industar-50" (both 3.5/50) were designed.

Characteristics: 
Lens s/number - 238186
Number of lenses - 4 in three groups.
Angle of view - 46o
Number of aperture blades - 8
Limit of aperturing - 1:3.5 (max), 1:16 (min)
Resolving power - (centre/edge) 32/22 lines/mm
Minimal distance of focusing - 1.25 m
Type of thread - M39x1/28.8 (LSM)
Dimensions (mm): 37*37*47
Focal distance - 51,39 mm.
Mass - 111 g