Taken from source:

During the course of excavations for the foundations of an office block at 3 Market Square Dover, sometime before 1967, a lead seal was found in a disturbed context. The seal was identified as a Papal Bulla of Pope Innocent III, 1198-1216.

A bull (bulla in Latin) was the name given in the Middle Ages to the personal metal seals used by popes and royalty to authenticate their documents. The word bull eventually came to mean the document itself. These seals are usually of lead, though occasionally of gold and were attached to the document by a red or yellow silk ribbon. These bulls were used up until 1878 when Leo XIII ordained that the old leaden seal should be employed henceforth only in bulls of major importance and be replaced ordinarily by a stamp on the parchment itself.

The one found on Market Square bears the usual design of the head of two apostles separated by a cross and surmounted by the letters SPA and SPE. The other side bears the name of the pope, this design first appeared under Paschal II (1109-1118). Although this style of portrayal of the heads was changing in the Renaissance, the design has not been altered. (1)

<1> Council for Kentish Archaeology, 1976, Kent Archaeological Review; Papal Seal discovered at Dover. Vol. 44 (Article in serial). SKE31753.