Cape of Good Hope 1855 4d Pair Rare Port Elizabeth Mail Bag Seal

1855-63 4d blue (SG 6a) pair sharing PORT ELIZABETH MAIL BAG SEAL CANCELLER. F/U, margins cut in places. Exceedingly seldom is a fully-dimensioned strike such as this example encountered. The canceller is rare in any representation. An important piece. The POST OFFICE/PORT ELIZABETH/CAPE OF GOOD HOPE mail bag seal cancellation known used on only ONE day 28th January 1859, was first researched & deciphered by Jack Shepherd in 1854. Why the mail bag seal was used in Port Elizabeth as a stamp obliterator is not known. Only TWO examples are known on cover.