The DC-6 was offered as either a day-plane, or a sleeper transport, or a combination with sleeping berths in the rear cabin. Some operators designated the rear cabin as first class and only had sleeping berths in there. If you look at historic photographs of DC-6 airliners, you can see that some of them only had the small upper-berth windows above the rear cabin windows.
The DC-6B had the same options, but I have only ever seen photos of DC-6Bs with the small upper-berth windows above the rear cabin windows.
The book “Douglas DC-6 and DC-7” by Harry Gann (Airliner Tech Series, Volume 4) has a lot of photographs showing the various combinations of windows, including a photo of a Pan American Grace Douglas DC-7B with upper-berth windows above the rear cabin main windows.