GAGB Chelsea Coloured Filter Oct 1951

The metal-cased Chelsea Filter, such as the round example shown in your image and marked by the Gemmological Association of Great Britain, represents a later refinement, introduced after the initial 1934 release. Based on Gem-A instrument catalogues, manufacturing styles, and surviving examples, the metal enclosure is generally attributed to the post-war period, most likely late 1940s to 1950s, when Gem-A began standardising and professionalising gemological tools for teaching, laboratory, and trade use.

The Journal of Gemmology ( October 1951) contains an advertisement listing a “Chelsea Colour Filter” in a metal moulding — and mentions pricing — showing that by the 1951, GAGB (Gem-A) has started offering the filter in this mountings.

This is a museum archives and no unit is available for sale at this moment.