Originally, GWR signal box levers had brasses with hand engraved numerals only which corresponded with further information on a brass or wooden backboard laying across the width of the frame.
Eventually, backboards were dispensed with and engraved brass plates bearing the lever number and function, together with the numbers of the leads (i.e. the levers that need to be pulled in order to unlock the lever) were attached to the levers
Some smaller boxes were given oval brass leads c1907, which may have been an interim measure between buttons with backboards and the familiar brass leads used afterwards. Oval leads were also used for outdoor ground frames and detonator placer levers for many years. It has been suggested that many oval leads were initially cast before it was realised that the small area available for the description was too limiting. (SRS Newsletter 92).
The use of red infilling for SIGNAL lever numbers appears to have been superseded by black c1912. (The Signalling Record 80)
Casting numbers and ribs cast on the reverse side appear to date from c1932. (SRS Newsletter 92)
Traffolyte, which was invented in 1927, was used for lever leads in the Bristol area power signal boxes of 1935. These were white characters on a black background. Black on white background was used in manual boxes from c1938 (SRS Newsletter 92. Does anyone have an earlier date for this?).