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 for >£1600

A double surcharge of green + black has not been catalogued

The double surcharge may be a forgery where a forged black surcharge has been placed on a stamp with a real green surcharge. However, the double surcharge may be real. The stamp is priced as a forgery