Cloth Seal, Dyer's Seal, Ox, Image by Derfel, Found by Andrew Bryson.
Found on Thames Foreshore, 25mm.
L E incused // ox standing on (grassy?) ground, DW above
A match has been found for this seal with one held in the Salisbury and South Wiltshire Museum (probably originating from the banks of the River Avon) No.123 Fig.24 in Egan, G., Cloth Seals, p.43-86 in Salisbury & South Wiltshire Museum Medieval Catalogue, Part 3, ed. Peter Saunders, 2001, but the incused letters are YE instead of CB on this one (a different junior dyer perhaps with DW being the senior dyer*). For another match see BSG.CS.00567.
*see BSG.CS.00673 and MOL 79.319/17, which both have a similar ox plus madderbags on their reverse.
From Egan, Lead Cloth Seals and Related Items in the British Museum, "incuse numbers are generally found on two disc seals of eighteenth-/nineteenth-century date - i.e. post-alnage."