ORIGINAL BRITISH EXPEDITIONARY FORCE LETTER

An original British Expeditionary Force Letter, 2 seperate sheets with writing on 3 sides, sent by a Gunner Hutton R.S.10/3 ( Ulster ) S/L ? Regt. R.A........  I'm presuming this to be a WW1 item as I can't find this unit designation in the 1939 B.E.F.........