Could Dr Gregg have been referring only to the externally visible canvas - the door tops and load area canvas?
I have looked through all my Ford parts books, and while they may have been printed too early for reference to No2 Brown (SE 39-42 Jan '42, F15A-01 may '43, FGT-01 Jul '43, F60L-01 Dec '42) they DO describe different canvas colours of Matte Green and Light Stone for the door tops, weathershields and tarpaulins, but not for the seats canvas.
In all the books, for all vehicles, the seat covers are:
C29Q-7864004 Cover, seat base cushion
C01Q-7860206B Cover, seat back cushion
There is no mention of what colour canvas these are, but I see that
Dirk Leegwater has these for sale, and they are clearly Matte Green canvas. (Pic attached) Are the Chev part numbers the same?