The Valley Of Fear

Holmes decodes a warning from Porlock, an
informant against arch-criminal Moriarty, for
"Douglas" resident five years at "Birlstone".
Scotland Yard's MacDonald asks them to
investigate a corpse with the same look and
circle-in-triangle brand on the forearm as
Birlstone owner Douglas. The head was blown
off by an American-style sawed-off shotgun.
Apparently, an intruder dropped a card with
VV341, and left across a shallow moat.