1991 26P FOR 28P ORDNANCE SURVEY MAPS ERROR OF VALUE (SG1579 VAR). A superb unmounted o.g. 1991 28p Ordnance Survey Map of Hamstreet, Kent (SG1579) but with the ERROR OF VALUE "26p for 28p". Very rare and one of approximately 20 believed to exist. SG cat. £5,000. (Ref 109134).