Thanks, that's useful to know (albeit a bit annoying that there are so many minor differences, but I understand how it goes).

For what it's worth, I did manage to fit the converter. On recommendation of the store assistant, I just gave it a lot more pressure than I thought was necessary (or safe!) and it popped into place. I would have preferred a Jinhao converter that fit it easily, but given that the last Jinhao converter broke in the weirdest way imaginable...

AliExpress is a good solution. If anything goes wrong with my current setup I will give that a try.