The black caps on the standard rail have a short plastic threaded male fitting (looks larger than 3/8") that could screw into either end of the standard rail. Your black cap should unscrew easily. The metal cap on yours would have to be on the extension tube and is connected to the rod that extends to the opposite end of the extension tube with the fairly large male fitting that would screw into the main tube.
I would guess that something loosened up or broke on yours. Someone "fixed" it by gluing the extension tube to the main tube. Normally they unscrew very easily.
The rod inside the extension tube is roughly 3/8" in diameter and is smaller than the male fitting that screws into the main tube. Maybe the male fitting came off or just came loose so someone glued everything together. I can't see how the male fitting, the rod, and the knob are attached together. There are fittings inside the extension tube that keep everything approximately centered, and block the view of how it all fastens together.
Both my main tube and my extension tube are roughly 32cm long.
jeff
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