The Great War prestige stamp book was issued along with the set of stamps on 28 July 2014.
The book is printed by Joh Enschede en Zonen and all of the definitive stamps have been printed by Enschede before, most recently in the Buckingham Palace PSB.
28 July 2014 - 20p green Machin definitive from the Great War prestige stamp book year code M14L, source code MPIL, printed by Joh Enschede en Zonen in lithography. Due to the printing process the coding iridescent ink is very difficult to read.
In the case of the mixed definitive pane, the phosphor bands on the pane from the Buckingham Palace booklet have a more matt finish when held at an angle to the light, the Great War ones are less visible. The iridescent layer appears to be easier to read on the Great War stamps.
There are shade differences on some stamps though these may be a normal variation in the printing, and will probably not have full catalogue status. In the picture of the stamps, the Buckingham Palace stamp is on the left and the Great War stamp on the right.
SG number U3014 as used for Dr Who & Buckingham Palace PSBs.