I am not sure that I have the right man, but man reseasonably sure on the grounds that the clues point to him being Irish. Plus the fact that nobody else in UK burdened their child with the name of either "John St. John" or "St.John John" . There are a number of Ancestry trees on this man, who man I detail below.There are no sign of a St.John John or a John St.John in WO338. There was no "intake" to ADRIC on 15April 1921, so his entry was different again pointing to Irish,
He is not on Numeric Register
1883 Jul 1. Born Earlshill, Tipperary
The Morman records show this man in "United Kingdom, Militia Service Records, 1806-1915," but no further details apart from the same place and date of birth. There is nothing on Ancestry or FindMyPast records for military
Married Margaret Langton
1921 Apr 15. Joined ADRIC as Assistant Paymaster at Depot Coy with service no 1923. No details of age, regiment.
1921 Apr 15 Appointed as Assistant Paymaster in U.D.R.S.C at £250 per year. He appears to have gone to Ulster Special Constabulary
1921 Nov 30. Struck off strength on ceasing to be employed as Paymaster.
1925 Mar 7. Died Glen Goole, Tipp.