Fowlers Long Scale
Fowler's double sided Long Scale Calculator, complete with velvet lined nickel plated case. Known also as a 'vest pocket' model, these double sided calculators were regarded as comprehensive and accurate. It measures 2.5 inches in diameter. Full instructions with worked examples can be found on www.sliderules.info/collection/inst-fowler.htm
D Scale length: 980 mm. Significant digits: 3.7 to 4.