Two shades of the SG151 on the left and middle. The left one with slight toning was issued in 1938 and the middle with fresh white gum issued with different paper in 1940 ( very rare) and the one on the right in the 141/2 X 14 SG 151a issued in 1945.

I suspect SG Cat £150 is for the one in the middle stamp which is much rarer than the stamp on the left. Good Luck 

Total Cat by SG at  £322+