This is a rare 1908 calculator called The Adder.  It is pictured and described in the Ernst Martin book.  The machine is a ten key adding adder used for the addition of single columns of digits.  Its operation is similar to the Adix calculator of the same period.  Top plate appears to be of copper with steel sides.  Bottom is stamped 7512; Patent 24868-1902.  It measures 6.5” x 4.6” x 4.5” and weighs 5 pounds.  All of the number keys work, but the zero key does not reset all the way back to 0.  The machine was manufactured in London for a few years and originally sold for two guineas.  Today it is a Holy Grail of antique calculators as few are found.    The machine has been carefully restored to a beautiful condition with new springs, glass and polish.  Collectors of antique calculators will find this a highlight to their collection.