I managed to find out some information on the СБМ20 Geiger Müller Tube.

I tracked down the following image on Google Image Search…

With the help of a Russian speaking coworker, I was able to find out the most important specs for this tube.

Nominal working current: 400 volts
Minimum plateau length: 100 volts
Maximum plateau slope: 0.1% per volt (10% per 100 volts)
Working range (min): 0.004 mkR/s
Working range (max): 40 mkR/s
Minimum life: 2*10^10 pulses

Hopefully this will help someone else.