That’s an excellent reference. I can tell you from personally firing Soviet/Chinese 107mm rockets, the tube is one side of the circuit, which directly contacts the rocket body, which is itself insulated from the base cap by a rubber gasket. the base cap has a wire that runs into the rocket body.
When firing from a launcher, the spring loaded retainers carry the opposite polarity as the rest of the truck/launcher. When that circuit is completed inside the cab the current goes through the body of the rocket and base cap into the igniter.
We of course launched them off of rocks, so we bypassed the whole system using a ten cap blaster - one lead taped to the body of the rocket, one to the base cap. Worked like a charm, as my video will show when I get around to posting more Afghanistan videos.
I had posted some here but apparently in looking at Youtube they did not get many views: