I picked up a bunch of CDV-700 radiation meters. Most of them are missing the Geiger tube. This is the tube:
Civilian gieger counter were the OCD-D-103 and the later issued high range OCD-D-101. The original tube is more commonly called the 114/6993 and the high range the Eon 5115. Both operate at a nominal 900 volts. The higher range tube was made to be exactly 1/10th as sensitive as the original and came in a box with a peel and stick label that corrected the range scale. No further modification of the CDV-700 was required to make the switch. Just plug and play.https://www.instructables.com/id/Homemade-High-Range-Tube-for-CDV-700-Geiger-Counte/
This link also discusses replacing the original tube with a Soviet geiger tube called the Si3BG that is plentiful and cheap and made to function up to 300 r/hr and will easily fit into a CDV-700 pickle probe. Runs at 300V, so Zener diodes are needed to convert it. link. Russian tubes, $13 shipped
The original tubes are rare. This site has them for $100. link
This site also has instructions for converting it to use a Russian tube. link
Info about CDV-700 era detectors
aWhat IS and what IS NOT a Geiger Counter. by George Dowell.