The initial 4c surcharge was in dark green. The later black surcharge is displaced to the left and down from the original surcharge..

The double surcharge is catalogued in either dark green and dark green SG19b CV £3000, or dark green and red SG19c CV £850.

The black surcharge is found on SG20 CV £1600. A double surcharge of black + black has been sold at auction.

A double surcharge of green + black has not been catalogued

Scans 3 and 4 provide a detail of the double surcharge plus examples of 4c on 8c surcharges in black and green and on other stamps in the 1898 issue.

The double surcharge may be a forgery. The auction start pricing assumes that the stamp is a forgery.