Hello David,
At a certain time point during the car's life a particular part wears out. Sooner or later.
That's the case with your shifter knob here. These parts are just not designed to to fine for 50++ years or so. They often do - for sure. But sometimes they don't.
If you had removed the know numerous times throughout any restoration tasks, the ball just wears out on the top of the shift stick.
Yes, your are right, there is no thread, it is just pressed on.
You have got 2 choices:
1st) (and best as to my guess) is get a new one:
A111 268 0942. €€ 56.00 in Europe; perhaps you can get a bit better quote from Tom.
Maybe Authentics etc. are even cheaper with repro shifter knobs, which are available as well.
With a car like yours (280 SE Convertible) I would always go with the original.
2nd) Tape the tip of your shifter with some tape to get a tighter fix of the old knob on the shifter.
Or glue it.
Both are temporary fixes - not suitable for a long future.
My 2 cents,
go with the 1st choice. Get a new one as long as those are available.
The old one will make a fabulous paper weight on your desktop.
Best,
Achim
