Nice, very nice piece of wood and, while the finish, or should I say, lack of it has been mentioned by others, has been a couple of identical pens you made recently that, look absolutely stunning as they are.
There is not often that I like a matt finish on a pen, as I prefer a deep gloss on pretty woods but, those of yours have that "velvet(ish)" look and possibly, feel to it that I like.
The best material that comes to mind as a close match is the smooth sanded raw cork...!
I don't thing that is a finish that will preserve that, unless maybe something like the Fungishield that I get here in Australia, that is mainly a wood soaking preservant and fungi repellent. It would soak through and then after dried (48 hours) would need very fine sanding to remove any shiny residue and make if matt again.
This would be certainly a process/step that, would be better done just before the barrel(s) is/are sanded to its final size, obviously...!
I don't have a product name replacement of something identical that you could by locally but, if you look at the technical info from the product manufacturer (Feast Watson) you could look/ask into a possible identical product...!
Hope this helps...!
Cheers
George