David,
It is a 1932 Douglas C32, or Bulldog. A 500cc side valve based on the 350cc crankcase, but fitted with bigger barrels and detachable plate heads. Possibly a B32 model of 350cc, but if the catalog illustrations are to be believed the A32 and B32 had a slightly different rear parcel carrier. Otherwise I can not think of any other way to tell the 350 and 500cc versions apart visually.
-Doug