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An ivory and silver coloured metal expanding paralle

An ivory and silver coloured metal expanding parallel ruler, mid 18th century, with pivoted scroll pierced x frame, two screws lacking, 11.5 cm long; with an ivory scale ruler calibrated as a protractor and stamped 'I: Sifson, London' to verso, 11.3cm long. (2) NB: The scale ruler is probably by Jonathan Sisson II c.1690-1747, or his son Jeremiah fl. 1736-88, Jonathan Sisson II was chief workman for George Graham and instrument maker to H R H the Prince of Wales.