UK traffic sign- Plate for use with signs in diagrams 504.1 to 507.1, 510, 512, 513, 516, 517, and 520;  diagram 511 (TSRGDNO 511), Outer Red border is to aid screen display, and is NOT part of the actual diagram); This is a deprecated design or diagram, reconstructed from analysis of the 1981 Schedule, and the 2002 era working drawings. It may not be wholly accurate.
Traffic Signs Regulations and General Directions 1981; Working drawings for Traffic Signs -  [1] namely the Alphabet Working Drawings here [2]

, and extrapolation of details thereof.; Redrawn by uploader.
Department for Transport (United Kingdom); Redrawn by uploader