There is the original seal of the Priory under Thos. Docwra held in the Westminster Abbey Muniments (13,601) and there are other ones in the Museum of the Order of St John (OSJ54)(OSJ55), of which the original is in the Westminster Abbey Muniments (9180). A different signet of the Prior is used as a counter-seal.
Two separate signets are known for Thomas Docwra, Turcopolier and Pillar of England from 1495-1501, and Grand Prior 1501-1527. The first is a Gothic Concave Shield, 5/8 x 1/2 inches, bearing the Prior's crest, "on a wreath f his colours, a demi-lion raampant, in its claw a plate, charged with a pallet". Above the crest are his initials "TD". This is used as a counter-seal (OSJ55) to the Great Seal of the Priory attached to a deed in the Westminster Abbey Muniments (9180). There is a specimen of the signet (OSJ51) in the Archive Nationale, Paris*, attached to a receeipt given by the Commissioners of Henry VIII, of whom Thomas Docwra was one, to King Francis I for the 50,000 livres agreed upon for the surrendour of a Tourney in February 1519.
The other signet (OSJ54) is also used as a counter-seal to the Great Seal of the Priory on a deed in the Westminster Abbey Muniments (13601). It is 11/2 x 33/8 inches and has a shield bearing the Prior's arms " a chevrn engraailed, between three plates, each charged with a star". [Note: A star and not a pallet as was the usual charge]
* There is a copy of the Seal of Grand Prior SirThomas Docwra is in the French National Archives [Ref: "Collection de Sceaux" par M. Douet d'Arcq, No. 10252, in the "Inventaire et Documents publiés pas order de l'Empereur", I Vol, ii, 1868]
Source: "Seals of the Order of St John of Jerusalem" by E.J. King, published Methuen, London, 1932.