That looks very much as if it has been made by welding 2 short sections of square ( ish ) tube onto the round shaft.

The square sections are then placed into each end of a longer section of matching profile tube.
The top of the assembly fully welded with the bottom section left with the ability to slide ( colapse ) in the event of a frontal impact.

Very similar way as the Sierra reach adjust set up in which a triangular section is used in the upper section of the column.

With a little luck that'll all make sense :) :)

seems like a good & safe way to make the extension , maybe even make the colapsible part longer. i will for sure be making mine the same way, it´ll also make it easy to take appart .
samy