The Northampton Roll of Honour from 1916 (from Northants Family Tree site) shows:
Charles William MARRIOTT, 77 CECIL ROAD, 7TH NORTHAMPTONSHIRE, SERGEANT
Stanley Walter MARRIOTT, 77 CECIL ROAD, ARMY PAY CORPS, CORPORAL
http://www.northants-familytree.net/soldier%20index%20roll%20of%20honour.htmThe 1918 AVL shows:
Marriott, Charles William, 77 Cecil Road, Cadet, Officer Training Corps, Pirbright.
There is no entry for Stanley Walter Marriott at that address.
It is sometimes possible to garner some information from the records of men with nearby numbers. That of Percy Thomas Wooldridge, who also served in the APC and then the MGC may be helpful in giving us some possible dates of transfer:
Percy Thomas WooldridgeEnlisted on 20-11-1914
A railway clerk
Posted to Army Pay Corps, No. 3733
Transferred to 96th Training Reserve Battalion, No. TR/7/17600 on 24-3-1917
Transferred to Machine Gun Corps, No. 103625 on 10-5-1917
To France on 10-8-1917
Posted to 54th Company M.G.C. (54th Brigade, 18th Division)
To Base Depot, 1-11-1917
Transferred back to Army Pay Corps, No. 3733 (as previously), on 29-12-1917
Class Z Reserve on 8-4-1919
We cannot however rely on the unit that Pte. Wooldridge was posted to being the same as that to which Stanley Marriott was posted.
I think that "Major General Kellett" is Major-General Richard Orlando Kellett, C.B., C.M.G. (14.1.1864 - 12.11.1931) who commanded 99th Infantry Brigade from 1916 to 1918 and then one of the Machine Gun Corps Training Centres from 1918 to 1919. Presumably the certificate is from his later duties so gives us no clues as to his whereabouts whilst overseas.
Steve.