VERY RARE usage. SG.51 cat £160 as stamp alone. Note GIBBONS state "-" as the multiplier for SG.51 on cover indicating it is almost impossible to find on cover. Addressed Bournemouth. Minor surface rub otherwise Very Fine for an item of this type & period. Lovely small clean cover. Please look at all the scans to help form your own opinion. Priced & sold with all faults considered.