Possibly the Imperial Service Medal, awarded to Civil Servants who complete 25 years meritorious service.
http://en.wikipedia.org/wiki/Imperial_Service_Medal The closely related Imperial Service Order entitled the holder to use the post-nominal letters "I.S.O.", so I presume that the the Imperial Service Medal might be "I.S.M.". I'n not sure where lists of recipients might be readily consulted but the London Gazette online archive would be a good place to try.