They are not chess pieces but Viking gaming pieces.
The Museum now display them on a Viking gaming board.
(as well as a chess set till the gullible tourists have bought up all the chess set stock.)
How do they know this was one of the so called Lewis Set.
(there is not one shred of evidence they were found on Lewis. Nothing at all.)
Trodenheim, Norway, where the pieces were carved is a good place to look you might find some more..
The British Museum bought the pieces at a knocked down rate because the Scottish
museum simply did not believe the story about a cow kicking sand dune and where they came from.
It was later discovered the pieces were all white and some have very recently been dyed red.
Asked why the sets were incomplete (the answer there is they are not chess pieces.)
Forrest (the owner of the pieces,) said he sold 5 or 6 bits to a private collector.
These pieces have never surfaced and Forrest refused to name who it was.
The pieces fortuitously appeared within months of the Edinburgh - London match finishing,
which was highly publicised both in England and Scotland.
The well known player and author, William Lewis(!) wrote a book about it.