GB Used abroad in Fernando Po, Duplex 247, s.g.146 6d Plate 17
 
Very few know and still not priced in the Stanley Gibbons British Postmarks handbook.
 
GREAT BRITAIN, Used in Fernando Po (abroad), 1874-77, 6p Grey Plate 17 (SG Z3).

Lettered P-J, plate 17, well-struck barred "247" cancel of Fernando Po, Very Fine strike, an enormously rare British used abroad cancel which has only recently been priced in Stanley Gibbons and the first example we have seen, unpriced in Scott.

Although it is listed in the Stanley Gibbons catalog, there are two few known copies to give an accurate catalog price!