The weapon(s) used in the crime are brass-framed copies of Colt's Single Action Army Revolver, aka the Peacemaker. Colt never made them with brass frames, showing them to be latter-day (i.e., 1950s or later) replicas, probably Italian-made.
Watson's revolver, which plays a pivotal role in the solution of the mystery, is once again a Webley Mk IV .455, a revolver not fielded by the British army until 1915, some years after the story is set.