The puzzle is to open the piece. It advertises M.C.L. AND REPETITION LTD. The outer surface also includes the words BRITISH ALUMINIUM BA-35. This will be the alloy type from which the puzzle is machined.
Cut the postcard into 3 pieces. Where does the 16th card go when the position of the two lower pieces are reversed? The postcard advertises Martinka & Co., a magic shop in New York City.