"In June 1916, 7th (Meerut) Cavalry Brigade (complete with V Battery, RHA) left the division and was reformed for service in Mesopotamia. Other than the Battle of the Somme in 1916 (Battle of Bazentin Ridge, 14–17 July and Battle of Flers-Courcelette, 15–22 September) and the Battle of Cambrai in 1917, the division was not involved in battle. Instead, it was held in reserve in case of a breakthrough, although it did send parties to the trenches on a number of occasions. They would hold the line, or act as Pioneers; such parties were designated as, for example, the Mhow Battalion."
https://en.wikipedia.org/wiki/XVII_Brigade,_Royal_Horse_ArtilleryThe only mention of Mesapotamia I can find with RHA, so was he V Battery RHA?
Forces War Records.com index has:
George Collin 51925 Driver 1919 Royal Field Artillery
George Collin 51925 Driver 1914 Royal Field Artillery
(Would the 1914 be attestation and the 1919 be discharge??)
https://www.forces-war-records.co.uk/searchHe had no middle name just George Collins? Did the medal Card not have another service number on it besides 51925? Did the medal card have a first theatre of battle? Can you search by birth place?
?? George Collin — — 1088300 —
https://search.findmypast.co.uk/search-world-Records/royal-artillery-attestations-1883-1942