Definition of "enamelled"

In recent issues two replies have been given to correspondents concerning the function of the diamond-shaped enamelled white plates fixed to signalposts, which are to indicate to enginemen that the section of line is track-circuited, so that action under Rule 55 of the Rule Book is unnecessary.

1943 March and April, “Notes and News: White Diamonds on Signalposts”, in Railway Magazine, page 116