I have seen a book called British Railways Mk 1 coaches by Keith Parkin, this is probably the definitive guide if you can get a copy from your library.
Here are a couple of interior shots, although from the preservation era, these are more or less as they would have looked in the late 70's and 80's The first has the original luggage racks, the original tables and the original window frames but the first incarnation of fluorescent lighting, the new seats (the originals had a solid timber wing rather than an upholstered one) and some Formica internal panels (previously all timber originally)
The second is has the newer style luggage racks, the newer style tables and newer style windows and more Formica.
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