I'll have to start by telling you that the Royal Artillery in it's
various forms was a complex entity.
In May 1916 re-structuring began & pretty much continued to be
until the end of the war.
The Great War Forum says this which will give you a taste of the difficulties
concerning the RA:
126th Army Brigade, RFA went out to the Western Front on 21 June 1917 with 2/A HAC, 2/B HAC, and 2/1 Warwickshire RHA with personnel only. On arrival in France the personnel of these units were attached to the following Divisional Artilleries in rotation as follows: 39th Division on 25 June 1917, 48th Division on 4 August 1917, 58th Division on 3 September 1917, 48th Division on 28 September 1917, 9th Division on 11 October 1917, 58th Division on 28 October 1917, 18th Division on 1 December 1917 and 14th Division on 16 December 1917. From 6 January 1918 to 3 February1918 the Brigade was at Calais, and during that period were being equipped with guns, horses and equipment. From 10 February 1918 to the end of the war the Brigade now equipped with guns served with various divisions in France.
Mind boggling isn't it.
One snippet I can give you is that his War Gratuity indicates he joined around July 1915.
I know your going to ask me how I know this so, War Gratuity was calculated at £5 for
the first year + 10s. for every month thereafter up to time of death.
His NOK received £13.