highly detailed entrance size doorknob backplate designed by ludwig kruzinger for the metallic compression casting co., boston, ma. the design was patented on june 7, 1870. manufacturer markings are incised on the backplate's thimble. the backplate retains the original swinging keyhole cover. nicely aged patina throughput. very little surface wear evident. metallic compression manufactured "house trimmings", such as knobs, hinges, escutcheons, bell pulls, along with dies for printing wall paper, and for bookbinders, and a variety of articles. the inventions of charles and michael smith, introduced with complete success into practical operation by the metallic compression casting company at their foundry in bomerville, near boston, revolutionized metallurgic art, so far as casting is concerned. by employing the patented process developed by smith and mason, metal was taken from the mould, not with a rough surface, as in all the heretofore known modes of casting, but as smooth and perfectly finished as a coin or medal struck by a die.