Sorry, got called away.
I think it has got something to do with these pencil sliders, and yes I think the nib is an addition, and I still think it is silver, it is soft. There is an opening at the end in which you could put pencil leads, but the hole that the nib fits into is very fine, presumably they would drop out of the slot. It is a puzzle. I think it is missing something, I thought at first it would have been a shaft that fitted in. Perhaps it is a "top" with chain like some of the others.
I agree that it is most unlikely to have anything to do with Mordant (I should be so lucky) and the maker M&C doesn't show up as a notable maker in my book. I suspect that it will go for scrap, at 3 grams, it's not worth the postage! I would still like to know what it is though. I suspect my father picked it up somewhere. He had ruling pens for his watercolour work. I wonder if he tried to use it for that.