GPO Seal, Image & Found by Donnydave.
Found in East Yorkshire.
From:Devon Panner (Thu 29 Jan 2009 15:19:45 EST) This is an old G.P.O. seal. (Square type).
In 1911 the post office replaced wax seals used in smaller post office regions with lead ones for sealing letter and parcel sacks (lead seals were used in the the larger post offices probably since Charles I opened up his royal mail to the public in 1635). In 1916 they made the seals thinner reducing the amount of lead by 19% thus saving £1,770 per annum. The Post Office Controller of Stores supplied the lead to the seal manufacturers directly. These manufacturers at the time were, Dunham White & Co Ltd., J.N.Lyons Ltd., The Lead Seal Manufacturing Co. and Walkens, Parker & Co. Ltd. (This information was gathered from The British Postal Museum & Archive, Freeling House, Pheonix Place, London.) Here are records of contracts for lead seals to from Acme Lead Seal Co. and The Lead Seal Manufacturing Co.
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.
