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I've got back to doing this job and here is the rest of the story, with a happy ending
As two connections to the lens are needed to complete a circuit, in the cases of those who have found that ensuring the
Data Pin 7 makes a connection to the lens is sufficient for the lens to be detected, I believe that the other connection must have been via the lens body to the camera body ("earth") in areas where the mount coating has worn sufficiently to bare metal to allow electricity to pass.
In my case however, the extension tubes, although sold as "used", looked as if they had never been used, and had no bare metal anywhere around the male mounts. The black surfaces of the mount must be some kind of anodising, not paint, and quite hard. I painstakingly scraped and filed a small area where Pin 7 touches, and for the earth I filed off the anodising behind one of the mounting claws; this cannot be seen in my photo. I removed the tube's mounting ring (four tiny crosshead (JIS ?) screws) while filing.
I have done this mod to only one of the set of three tubes, the thinnest. As long as this is fitted nearest to the camera body, the arrangement will remain functional with the other tubes mounted outboard of it. I think that should cover all use cases for me.
The photo shows the extension tube's male mounting ring with the bare contact patch for Pin 7 on the right. Having now dismantled and worked on it, I am impressed by the build quality of these Vivitar tubes.