This relates back to the Pelikan entry of a few days ago. As suggested, I contacted Rick Propas and he very kindly replied almost immediately. Here’s what he had to say:
Always hard to tell just from images, but it looks to me like an M250.
Put it up against your M400 should be the same size, if not it could be
an M150 fitted with a gold nib. Regardless, nice piece.
Taking size into account, it looks like it’s an M250.