@Esme866 Sorry, it was midnight when I had the inspiration to get the Bernina out and try it. What I actually did was sit it on the under pin, which is smaller.
When I woke up, I realised and went to try it on the proper tall pin that sits on top. The adapter slides on to that on but still a bit loose, but stops
just before coming out of the tapered end. It would probably fit snugly if I forced it that last mm. but I didn't want to break it. It is supposed to go on the post tapered end first.
I would measure the Featherweight post for you but I don't have calipers that accurate - it is only a squeak.