I used to own this knife, God knows when. The walnut was shop-supplied...and installed. The checkering and inlay were done, after-the-fact and I do not know by who (whom).
Joe is correct about Tom having the approval of Bo to customize RMKs however he liked but there were a couple of others who come to mind also, namely: Gail White & Tommy Bish. This handle was done by none of the 3 mentioned here.
Hope this helps, Capt. Chris