Hi Speedy,
Some of the Homelite and Ryobi straight shaft bump head can be identified if left or right hand thread with the colour of the bumb head. Red is a right hand thread and black is a left hand thread. Most or all straight shafts are right hand thread and most or all bend shafts are left hand thread. This is due to counter centrifugal force that tightening of the bumb/screw head is against the direction of the rotation of the head. This is opposite direction for the straight shaft because of the gear box that turns the head in opposite direction with the shaft/engine. I can confirm the bump head is a left hand thread. Turn CCW to undo it. Could be seized with rust in the thread.
Cheers,
TheCarbyMaster
