as the knobs are riveted to the handle, way back in the day, i had accumulated a bunch of them because they would bind up and break the rivet.
i think that was because we just cranked on them in a haphazard way, and with the slightest bind, the rivet was the weakest link.
we sold lots of those handles back in my junkyard ownership days, 1972-1984.
over the years, the ones i had squirreled away became missing............
i don't believe the knobs are sold separately.