Possibly awarded for Battle of Britain campaign?
Usually the clasp would have something written on it to denote which campaign the medal was awarded.
http://www.defence.gov.au/medals/Con...ar/default.htm
http://en.wikipedia.org/wiki/1939–45_Star
If you look under Australian Imperial Awards you might have some success otherwise contact the Australian War Museum (AWM) Canberra and they might have knowledge of this clasp.
You can request your Grandfathers service records from archives which will list his awards.
http://www.naa.gov.au/collection/exp...rvice-records/