The Household Cavalry is not an area I have much knowledge of ,so I decided to look at a thread by RogerV Household Cavalry and was surprised to see a reference to the Household Siege Artillery . As ever the Pals pointed me in the right direction, a quick look at the Long Long Trail, Google and Frederick' Lineage book.
Starting with Frederick's Lineage, the Royal Garrison Artillery Siege Battery listing excludes 520 Siege Battery....so look at the Household Cavalry. Obviously they were not seen as an integral part to the Royal Regiment.
520th Siege Battery R.G.A. Formed by 11.17 from Household Cav pers, disb 1918 , pers intended for 141 Bty R.G.A which was never formed.
Spot of Googling...
520th Siege Battery embarked at Southampton on the 31 March 1918 and disembarked at Le Havre on the 1 April 1. On the 11th November 1918 it was an unbrigaded battery as part of First Army Artillery. They were equipped with 4 x 6inch guns.
In 1918 they were commanded by John Jacob Astor 1st Baron Astor of Hever. A member of the Waldrorf Atsor family, gold medalist in 1908 Olympics, and lost a leg at Cambrai in 1918
Additional information: 520th Siege Battery RGA