S23 – Cape of Good Hope cover piece with 4d pale blue of 1865 Dispatch King Williams Town 1866 Receiving cancel Grahams Town 1866

Cape of Good Hope. Early, large piece with 4d pale blue of 1865. Must be special otherwise a 1d stamp would have done? Pale red oval dispatch cancel from King Williams Town, 4 May  1866 over which is the receiving cancel of Grahams Town, Cape Colony, May 5 1866. The cancels were originally on the reverse of the cover. Please see photographs.