Not all the coaches used would have been of G.N.R. origin, East Coast Joint Stock coaches were used for through trains up and down the East Coast line an example is linked below , the text would seem to be the same as used by the LNER in the 20's or 30's . I admit I am not an expert on the subject despite living on the edge of what was the GNR's territory!
http://www.nrm.org.uk/OurCollection/LocomotivesAndRollingStock/CollectionItem.aspx?objid=1988-7008&pageNo=2&cat=carriages&comp=All&ipp=12
https://www.flickr.com/photos/48500638@N04/6195489468
http://www.cs.vintagecarriagestrust.org/se/CarriageInfo.asp?Ref=863