Hi Brian, great question.
Chris is exactly right, S on a medal to Scottish regiments denotes war time enlistment.
Below are the other S prefixes -
S - Royal Naval Volunteer Reserve Sussex Division
S - Army Service Corps Supply Branch
S - Scottish Regiments Wartime Enlistment
S - Army Service Corps Supply Branch
S - Royal Army Medical Corps
S - Royal Artillery
S - Highland Regiments Wartime Enlistment
S - Home Counties Regiments 3rd Battalion
S - Rifle Brigade
S - Royal Munster Fusiliers
S - Army Ordnance Corps
S - Dorset Regiment
Cheers, Dan