When on duty, Royal Signals personnel wore a pair of blue over white armlets on their upper arms to show they were engaged on official business. A fuller explanation of their issue and use comes from Brian L Davis’s book, British Army Uniforms and Insignia of World War II:
A free issue of two signals armlets was made to officers engaged on Army Signals Service to Royal Signals officers and regimental signalling officers; to Royal Signals NCOs in charge of regimental or battallion signal troops, sections or detachments, Royal Signals; to dispatch riders, Royal Signals; to motor-cyclists of other arms while employed on despatch rider duties; and to Royal Signals personnel engaged in the construction and maintenance of signal lines. The horrizontally divided white over blue armlets were worn on each upper arm and only during actual duty. They were worn when on active service or during unit or formation training but were not worn when there was a danger of the armlets attracting enemy fire to the wearer. These armlets were worn for two purposes: their use denoted persons engaged on urgent inter-communication duties and so ensured they were not unduly delayed or hindered in carrying out those duties; and they enabled key inter-communication personnel to be immediately recognised.
This armlet then complies with the regulations and has two broad stripes of white over blue:
The armlet secures on the rear with a pair of brass press studs:
Three different settings are provided to allow for different circumferences of upper arm to be accomodated:
Here a member of the Royal Canadian Corps of Signals can be seen wearing the armlet: