This is another glass russian Geiger tube made for professional use, not for military like all the M-series tubes. The MS-6 is gamma-selective and have a copper cathode. The internal wire is the anode and is made of platinum.

MS-6 tube

I’ve found two different but similar specs for this tube from two different eBay sellers

First eBay seller spec:

 ·         Radiation determine:   (γ) gamma soft/hard
 ·         Voltage of start of counting: 720 - 780V ;
 ·         Operating voltage (recommended): 820 - 880V;
 ·         Length of flat part of counting curve (plateau):  200 V
 ·         Slope of the counting curve:   < 0.1 %/V;
 ·         Sens. background:   < 120 counts/min;
 ·         Maximum radiation: 5*10e4 counts/min
 ·         Load resistance: 8 - 15  MOhm
 ·         Counter resistance: 4 - 8  MOhm
 ·         Allowable stray input capacitance: < 25 pF
 ·         Working temperature range: -40…+50 C
 ·         Anode material: Platinum (Pt)
 ·         Cathode material: Copper (Cu)
 ·         Dimensions: counter length 260mm, diameter 21.5mm

Second eBay seller spec:

Beginning of the counting voltage - 720V-780V.
The working voltage interval - 820V-880V.
The length of the plateau of the counting characteristics - not less than 200V.
The slope of the plateau of the counting characteristic - 0.1%/V.
The maximum count rate - 25000imp/min.
The biggest overload irradiation - 50000imp/min.
Sensitivity (when irradiated with 0.1 μR/s) - 4500imp/min-5500imp/min.
The largest background - 120imp/min.
Service life - 3∙108imp.
Temperature - from -40ºС to +50ºС.
Dimensions: diameter - 23mm  length - 266mm Weight - 65g