John, as I remember it, there was a torque arm attached to what might be called a rockshaft (6" diameter tubing?) to which the platform support wheel was attached at the outer end. The spring assembly, of course, attached to the lever arm to counterbalance the header weight. There was a much longer torque arm(s) that went from the rockshaft to under the header to carry it. There was some kind of heavy duty u-joint at the inner end of this rockshaft to let the header flex up and down.
You've really got me wound up now thinking back 50 or 60 years! What I wouldn't give to see this in the flesh today. Here is a shot of a restored #17 painted green. Ours was not painted but rather galvanized. Something I remember soundwise is riding the operators platform where the loudest noise was the four cylinder Hercules except when the governor opened a little on the Cat RD6 and the "three cylinder cackle" would come thru.