Who was the maker of my old hammer dr signed "John Rigby & Co. Dublin & London"? A known one?

Well, Springer's Erben (=inheritors), Vienna, Austria, is the best known and most posh of all Austrian "name" gunmakers! Before 1918 they were "Gunmakers to the (Austro-Hungarian) Imperial Court. They are CIB. http://www.springer-vienna.com/