The example below is PP24a a 1 1/2d. lake-red with error of lettering OP - PC for CP - PC (SG 51 or G6c) Plate 1 with a Copestake, Moore Crampton & Co., London type 12 underprint.
As a basic stamp the OP - PC error of lettering is quite rare. Combined with the Copestake underprint it is very rare indeed. I have only seen reference to one other copy.
I have a B.P.A. Certificate from 1954 which came with the stamp when I purchased it some time ago.