General Post Office postman's badge

Postman's brass badge with black enamel marked GPO 303 IV. Stamped on back "A reward of 1/- will be given to the finder of this badge on handing it i at the nearest post office". With two eyes on the back for fixing to the lapel? This was in the old Dornoch post office as an exhibit while it was used as a tourist and admin office by the Highlands of Scotland Tourist Board.
Picture Added on 18 February 2009.