I’m enjoying writing with the TWSBI 530 a lot more after installing a Pelikan nib on it (likely from a M250), though with the clear faceted barrel I can’t shake entirely the feeling that I’m writing with a Ferrero Rocher box. Switching nibs was easy. The TWSBI nib and feed are friction-fit and can be pulled out. Here’s a demo on YouTube:
On a side note – the original EF nib on the TWSBI wrote a very fat line closer to a Fine-Medium, with a little feedback.
EDIT: I originally thought the nib was from a Pelikan 140.