The LMS, LNER and SR introduced upper quadrant signal arms in the late 1920s. At first, the arms were of the corrugated steel pattern, but this changed around 1936 to the plain type. These signal arms were used for all new installations, and also gradually replaced the lower quadrant arms on signals inherited from the relevant pre-Group companies (a process that is still not complete!). BR adopted the design, and new semaphore signals are still being installed today, where replacement by colour lights is inappropriate.
All three companies began by installing the arms on wooden (SN17) and lattice steel (SN2, SN22, SN42) posts and dolls. Concrete posts (not yet in the 2mm scale range) were also used, particularly on the LNER. In the 1930s, the LMS and LNER switched to tubular steel posts (SN30), whilst the SR began making posts from bolted lengths of scrap rail. BR continued these designs.
This is the original MSE fret, since superseded by the ex-D&S frets of SN12/2. It does include some parts not on SN12/2, such as a sighting board and ground signal disc faces.
The actual size of the fret is 42½mm x 27mm; it is etched in 0.008″ brass.
Instructions are not provided – see those for SN12/2 instead.