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This 1930s Add-O-Matic Calculator was manufactured by Allied Manufacturing Company or Chicago, Illinois in 1937. It is complete with its original box and instruction manual and is in excellent visual and operating condition. 

The Add-O-Matic utilises slider knobs each with a pointer delineating a larger figure for addition and smaller complementary number for subtraction. Once a slider has been moved to the desired number that number is selected by pressing the brass button and drawing slider back. The results are displayed in the lower window register.