White metal (sometimes referred to as German silver) pre dates chrome plating. As a general rule of thumb, chrome plating began to be used on police insignia circa. 1935. A Home Office police clothing committee report was published in 1934 which recommends (amongst other things) that insignia should be of a particular size and design and that it should (with one or two exceptions) be chrome plated (or in the case of certain helmet ornaments) be black.
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