Well, here I go paddling upstream again...Have to disagree...Bryan
Me, too, now, Sir! ;)
I've owned exactly that hihat back in the 70's, bought brandnew and directly at a dealer, together with my Royal Star drumkit. It's the 'Hi-ace'-version with adjustable spring tension by turning the corrugated knurl at the bottom of the stand, just on top of the pedal. The pedal itself was linked to the rod with a strap made out of steel.
The simple explanation is: at yours, someone has replaced the original rod with this hexagonal one. Although I never faced problems with a slipping clutch, here someone must have decided that a hexagonal rod should work better. Replacement should have been easy, as the Sonor thread apparently is the same as the one from the bottom Star rod.
Furtheron I've just contacted a guy at the German Tama representation, having, experience with their drum line back to the Star days. He, too, can't remember that Star/ Tama ever used a hexagonal rod at any of the hihat stands.
Ralf
NB: the shape of the cymbal stands say that these ones appear to be Star-made, too.