1895 Aug. Born USA on RIC records. The only man that fits this in USA census data is born Pennsylvania. His father Silas is a Blacksmith, and his mother Martha was born in England. Though I doubt that this is he, as he is down as "Wilfield R Moon" in 1900 and "Reuben Moon" in 1910 when that family are at Jermyn Ward 1, Lackawanna, Pennsylvania and the father is now a coalminer.
His arrival in UK from Chile puts him down as British. In addition the chap arriving from Chile in Dec 1914, seems reasonably upper class to be traveling 1st class and going into K E Horse and getting commission
1914 Dec 28 Arrives UK from Buenos Aires on SS Highland Rover. He travels 1st class and his last country of residence was Chile. He gives his work as "clerk"
Enlists in King Edward Horse. On the outbreak of war in 1914, the regiment was mobilised in London, where it remained until April 1915; at this point, the regiment was dispersed, and the four squadrons were sent to the Western Front with separate divisions.
1915 May 4. Lands in France
1915 May 29. Enters hospital - "secondary syphilis"
1915 Jul 27 Discharged from hospital
1916 Apr 30 Commissioned into Buffs
1917 May 13. Arrives back in France
1917 Jun20 Back in hospital with syphilis
1917 Jul 12. Leaves hospital, back to base
1917 Aug 6. O/C 223 coy at its formation
1917 Sep 8. Posted to 223 RND
1917 Dec 21 till 4 Jan 1918, on leave in UK
1918 Mar 1. The undermentioned to be actg. Majs while comdg. a Co. Temp Lt W. A Moon
1920 Jan/Mar Marries in Romford, Essex to Ethel Richardson (b1899). He is on voters list in London in 1918 and 1920, then there is no further mention of him in UK records. The address on his MIC of Courtland Av, Ilford is the home of his wife Ethel , dau of Annie Stone and stepdaughter of Henry John Stone, a commercial traveller.
1920 Jul 20. Joined ADRIC with service no 32. Posted to A Coy Kilkenny
1920 Nov 18. Two members of the ADRIC stationed at Woodstock, County Kilkenny, who are on duty in Cork, were attacked by a crowd of ten or twelve men at Parnell Place. Captain Jones was knocked down and deprived of his revolver. Major W A Moon fired five shots, and the attackers fled. Two of the assailants are believed to be wounded. The two officers escaped without injury.
1921 Aug 12. Re-engaged by ADRIC on further contract
Fined £3 by Coy Commander
1922 Jan 18 Discharged on demobilisation of ADRIC
1927 Jun 27. Drink drive charge
1933 Driving accident. (another paper has it using is full name)
I cannot get a UK death record