First appearing in 1887, these wooden arms, on wooden posts, were the most common style in the heyday of the Great Western Railway. From the 1930s onwards, they were replaced by the new steel arms (SN15), at first on the original posts, and later by steel arms on tubular steel posts. Some are still in use today on the various preserved lines. Note that the 5′ arms are for use on posts greater than 26′ tall.
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.
To model a complete signal similar to the 4mm scale one shown, you will need a post (SN17) and a ladder (SN9/3 or SN9/7). Other parts such as the finial, lamp and platform, will have to be scratchbuilt.