Cloth Seal, Norfolk / Norwich, Alnage, Castle, Image & Found by London Mudlark.
Found on Thames foreshore.
Three towered castle // Rivet head has traces of a crown
The arms of Norwich but no C N to sides or lion at the base can be seen, also no inscription visible. If lion was absent then this would be a Norfolk County alnage seal as opposed to a Norwich City one, however the absence could easily be due to a weak striking or wear.
See p.43, Egan, Lead Cloth Seals and Related Items in the British Museum, Occasional Paper 93.