This pattern of steel arm was introduced by the GWR in 1930, replacing the wooden design (SN14) which had been in use since 1887. These signal arms were used for all new installations after 1930, and although usually associated with tubular steel posts (SN30), also gradually replaced the wooden arms on their original wooden posts (SN17). The design was adopted by BR and its successors, and a very few lower quadrant semaphore signals are still being installed by Network Rail today, where replacement by colour lights is inappropriate. A few signals combined the new pattern arm with the old pattern spectacle plate.
The actual size of the fret is 42½ x 27mm; it is etched in 0.008″ brass. For clarity, the photo shows the equivalent 4mm scale fret.