How does the “Gear shift lever ball grip” look?
If it is not threaded probably it is the same as on my 1922. Part #H332 in the Parts list.
I cut a piece of armoured rubber mat, 1.5-2 millimeters thick, punch a round hole in the middle, large enough to press it over the ball on the lever. Then adjust with the spring loaded (?) screws. I guess you have the springs to keep the ball grip? When driving, keep an eye on the screws that holds the ball grip. As the rubber mat piece gets worn, maybe you should tighten a bit before the lever gets sloppy.
For me it works fine say 500 miles, maybe more. Then I have to make a new one.