Industar 50U-1 red P
lens for enlarger, #5707
50 mm, 1:3.5, M39 mount
Lytkarinsky Plant of Optical Glass (LZOS), Lytkarino, USSR

Good condition. Front and rear glasses have a bit dust dirt at the edges and cleaning microscratches and a fungus dot. Inside there are microspecks of microdust. Body has signs of age and use. Aperture ring has a small backlash and rotates a bit tightly.

This is a lens for enlargers for frame size 24x36mm. Difference from previous "Industar-22u" - more precise registration of photo and visual layers. 

This lens was produced at three plants: LZOS (Lytkarino), KOMZ (Kazan), NPO Optika (Moscow) since 1954. I-50u-1 has much more resolving power that usual I-50u (60/20 lines/mm vs 45/20 lines/mm).

Note that lens is for enlarger so it does not have focusing ring. Focusing is achieved with moving of the enlarger body. If you want to use it on camera you need macro rings or extension bellows. 

Thanks to its resolving power and sharpness it is now often used as macrolens.
This lens is very-very small and lightweight. You can easily put in your pocket.
Lens has one-layered antireflection coating.

Its price in USSR was only 4 Soviet Roubles (with average salary about 120 Soviet Roubles per month), so it was very easy.

Characteristics:
Lens s/number - 5707
Number of lenses - 4 in three groups, Tessar.
Angle of view - 38o
Number of aperture blades - 8
Limit of aperturing - 1:3.5 (max), 1:16 (min)
Resolving power - (centre/edge) 60/20 lines/mm
Minimal distance of focusing - 0,3 m
Type of thread - M39
Type of thread for the nozzles -  cannot be mounted
Dimensions (mm): 42*43*43
Focal distance - 53,4 mm
Mass - 72 g
One-layered anti-reflection coating