Leave book has a "RM Williams" in E Coy and serving till demobilisation. RIC have him as Rowland Williams born 5 May 1895 in Middlesex
My best guess is from WO338 only Rowland Myrddin Williams of KSLI fills the clues . This is certainly the man , all the clues crosscheck including dob.
1895 May 5. Born Hendon Middlesex
1901 census . Living at Radlet, Herts
1911 census at Cranbrook Watling St Radlett, Aldenham, Hertfordshire
1915 Aug 14. Commissioned 2nd Lt in KSLI, 9th batn
1916 May 1. Transfers to R W Fus
1918 Jul 6. Transfers to RAF. -- R.W. Fus. Temp. Lt. R. M. Williams resigns his commission on appt. to R.A.F
1919 May 21. To Unemployed List
1919 Jul 17 Became a freemason at Elstree
1921 Feb 16 Joined ADRIC with service no 1711 . Posted to K Coy
1921 Mar 13 Posted to E Coy
1922 Jan Served until demobilisation of ADRIC
1927 Feb 26 Rowland Myrddin Williams, late TA to be Lt in RW Fus
1939 Register. At 1 Drapery Mor , Cowbridge R.D., Glamorganshire, with wife Evyln. He is Lieutenant R W F Regular Army Reserve & Assistant Station Adjutant R A F St Atham
1975 Mar. Died in Yeovil aged 79