Post Office GPO Square Seal, Southampton, Image & Found by CoinhunterUK.
Found in East Dorset, 9.6g.
Standard General Post Office square bag seal with grill of five vertical bars on both sides and on one G.P.O.SOUTH- above and AMPTON below. A definite Southampton Post Office letter sack seal.
Oliver Cromwell established the General Post Office in 1657. For an overview of the history of British postal history from Roman times onward see here.