Thinking about this I decided to check the actual parts to make sure I'm remembering this correctly. When I received the car (used) it had an aftermarket shift knob (a #3 billiard ball!) which has the M10-1.25 thread. The original Morgan supplied knob, which I have, appears to have no threads at all, just a rubber internal sleeve which is approximately 10mm in diameter. I don't have the original shift lever which I swapped out when I converted to a forward shifter position years ago. My current shift knob is M10-1.25 thread.
So I can't definitively say how your gear knob is retained, as I don't recall what it took to remove the original shift knob. But a threaded knob will have thin lock nut under the knob which keeps it from spinning or coming loose.
Sorry about the confusion.
Edit: an update with a video link to Phil Bleazy doing an engine removal. The bit about removing the shift knob begins about 23:30 minutes in.
