[SunSoaked]

As you know the shaft has a inner and outer sleeve that permits the slip fit. If I recall the overlap of those shafts was quite generous like 6-8 inches. The length is set by the plastic that is injected at the factory. You have to melt that plastic to allow for length adjustment.
The setscrew is what takes the function of the missing plastic. Although it won't really fix the length it keeps the two pieces from rattling and getting loose and sloppy. I never did this with mine and it's nice and tight but, it will loosen up with time. I will spot weld a nut to the outer shaft over a 1/4" hole so I can install a short bolt. I doubt there's enough meat on the outer shaft to hold a setscrew by itself. You don't want it going thru both shafts preventing it from collapsing in an accident.