From Computer Museum report, Summer 1983: "Manufactured by J. Halden and Company, London, during the 1920s, the pocket watch style calculator and slide rule has two logartithmic scales, A and B, on one side for multiplication, division and proportion which correspond to the A and B scales of an ordinary slide rule. Around the outer edge is a scale of logarithms read by a cursor. The inner circles contain a scale of square root. The other side of the Calculex contains A and B scale for inverse proportions."
