Post Office GPO Square Seal, Southampton, Image & Found by Coinhunteruk.
Found in Dorset, 18mm, 9.9g.
Standard General Post Office square bag seal with (G.?)P.O.S(ENT?) above and (A)MPT(ON?) below. A Southampton letter bag seal - see other Southampton seals.
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.