NEW (NOS) SBM-20 Geiger-Muller counter tube.
Tested and fully working.
One counter in one lot.
The counters may have different production years and slightly different appearance. 
Some tubes may have tiny dents on their metal surface.

The Geiger-Muller counter SBM-20 is designed to measure hard beta and
gamma radiation for wide application in the field of radiation monitoring.
Technical characteristics of Geiger-Muller counters SBM-20:
Radiation source - Cs;
    Sensitivity to gamma radiation:
    EDR - 4.0 µR∙s-1;
    67.5 µR-1±7.5 µR-1;
    The rated operating voltage of the device Geiger-Muller counter SBM-20
is 400V;
    Counting voltage - 260V-320V;
    The length of the counting characteristic is not less than 100V;
    The slope of the counting characteristic is no more than 0.1%/V;
    The intrinsic background of the device Geiger-Muller counter SBM-20 is
no more than 1.0 s-1;
    Maximum operating DER - 1400 s-1, 40 µR∙s-1, k.n. ±20%;
    The maximum permissible DER is not less than 360R∙h-1;
    Dimensions - 11x11x108mm;
    The weight of the Geiger Muller counter SBM-20 is 10g.