An original 1920s Darnley's Patent Rotatable Lightning Calculator combining a pencil case and ruler in one ingenious advertising and calculating device. These novelty calculators were designed with a rotating inner scale allowing rapid calculations and conversions, making them both a practical office tool and an interesting example of early twentieth-century engineering. The calculator rotates smoothly within the outer case and incorporates a ruler along one edge. It shows the expected signs of age and use, including light surface wear, marks and minor rubbing consistent with a genuine item approaching 100 years old. Please study the photographs carefully as they form an important part of the description and condition report. A scarce and unusual piece of vintage calculating equipment, appealing to collectors of scientific instruments, office antiques, patent inventions, slide rules, desk accessories and early advertising ephemera.
 
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