I took the knob off of my 10K and it is threaded 7/16" x 20.
It seems odd that the 10L would have a beefier clutch but the knob is attached with a smaller thread than the 10K. It would be interesting to know if that is factory or if that was some repair/retro-fit. When OEM parts dry up, anything is possible although putting the small left hand thread into a replacement shaft would require some special attention. I'm sure left hand taps can be bought, but around here it would have to be a special order.
The knob appears to be cast and where it runs against the collar around the shaft there are wear grooves. Collar is definitely harder than the knob as it shows no wear. It works smooth enough but were I to make a new knob or find the need to face the back side of this one flat, I would consider using a wear washer between the knob and collar.
At any rate, it should be easy enough to spin up any style of knob that suits your fancy. Heck, the hardware store is likely to have several styles of plastic knob already threaded 3/8 that you can use until you get the one you want.