With those doodles in mind, I applied an abrasive cutoff wheel to the shaft of an inspection mirror (from the usual eBay supplier) about 15 mm behind the second joint. That puts a short section of the third tube inside the yet-to-be-built helmet mirror mount.
The two copper-colored springs center the smaller tube inside the larger one and provide enough friction to make the whole thing work. The tubes seem to be chrome-plated brass and the springs might be phosphor bronze. I suppose they’re Matryoshka-sized from one end to the other.
I’d never taken one of those shafts apart before; now we both know.