[Arran]

 I don't own one of these sets but from what I read the earlier tombstone version used a two speed tuning mechanism where you would pull out the knob for fine tuning, all the later versions of the 16/116 used two concentric knobs with the center knob used for fine tuning, so it would make sense that it would have a set screw to hold it on the shaft for the earlier type. I couldn't tell you whether the large hex knob with the set screw or the smaller one with the tapered back and and set screw is the proper knob.
Regards
Arran