Circular slide rule with pouch and instruction booklet, Halden Calculex calculator, duplex, metal / crystal glass / leather / paper, made by J Halden & Co, Manchester, England, 1920s
slide rule: Dims: 60 diam x 15
case: Dims: 80 x 70 20
concentric disks with milled winder
Obverse: outer: 0 - 100, A | B (1 - 10), C split (1 - pi, pi - 10)
Reverse: A | B Reciprocal | C split Reciprocal
Mfr: J.Halden & Co., 8 Albert Sq., Manchester.
Source: Bermondsey market, London. Date: 1981 / 2 / 20
"Circular pocket slide rule of metal construction. 2 ½ inch diameter. Two faces, each glass face acting as a cursor. Complete in soft leather pocket pouch with instruction booklet. Excellent condition."