Gas test - AVO VCM mk 1

[pwdrive]

Hello all, Interesting circuit, some similarities to the set up in the Hikock testers, I have a passing interest as to the patents that the valve tester manufacturers took out in order to protect their interests, is this something you or anyone would have any info on?
The PDF below is of a Hikock designed tester, see what you think, the best description etc is further down in the manual.

Lawrence.