The original 52-5000PUR drag knob currently fits only the original 39-5000PUR and 39-6000PUR shafts.
There is a difference between what shaft is in the early run reels and what Penn put on the schematic.
You can visibly see the difference in the threads, and the drag knobs ARE ALSO DIFFERENT from what was on the reel originally. Penn made a design change.
There are distinct threads for part type:
SG parts are COARSE threads.
PUR parts are FINE threads.
Your Pursuit reel could have EITHER type in it.
See the link to the design change notes for additional information and assistance with ordering parts.