It is a useful book, but it is a history of the RE Signal Service so other signals organizations like that of the RFC are only periphery to his story. He does mention borrowing ‘Stirling’ sets from the RFC to use as ‘Trench’ sets in June 1915 (page 86). It is actually ‘Sterling’ after the Sterling Telephone and Electric Company of Dagenham, who undertook initial production.
The Sterling set first appears to be used in action at Neuve Chapelle in March 1915, and became ‘standard’ by the autumn of 1915. However, it was in 1916 with the Somme that wireless was used in large numbers (306 aeroplanes fitted). Volume 2 of ‘War in the Air’ has some information on its introduction.