Signed Calvert sets are hard to find. Thomas Calvert, a Liveryman and Master of the Worshipful Company of Turners, operated from premises at 189 Fleet Street, London, from the end of the 18th century until his death in 1822, at which point his wife, Dorothy, took over the business, running it until her death in 1840.
All four rook stamped: CALVERT...189 FLEET STT