This Brunei black overprint error was spotted at the recent Cherrystone auction.
The error is the black overprint on the right stamp as opposed to the ‘normal’ red overprint. 1906 1c black & purple, two examples, one with error overprint in black, tied by blue Brunei 31 Jan cds on small piece.
The stamp with overprint error is Brunei’s SG11a and has a catalog value of £3,000. According to Stanley Gibbons, only one sheet of the 1¢ received the black overprint. Sold for US$1,400.