Hello Mike - only my opinion, but I suspect not BT - I think we would have found a reference to their factory design had it been from them.
this most recent of your examples of the 'empty book' design has an almost buff appearance, although that might simply be how it's coming across on my screen, but it certainly doesn't have a white look to my eyes.
I have further BT factory drawings from my trip to Kew earlier this week, which I'll post here eventually, but not aware that this design is amongst them -regret to say.