SkipD wrote in post #15833627
...The connectors in lenses or properly built extension tubes are smooth gold-plated connectors with little springs behind them for proper contact tension.
As writen above this is not the end result. The bolts need to have rounded edges.
In my very well working extension tubes (not Canon or Kenko, naming rubbed off after several years) there are no springs, and no gold plating.
I think the Kenko 11 or 12 mm one is not very different.
But as said, the connector bolts need to get a better smoother finish.