With all the speculation going on, I thought it made sense to just ask Dennis Ritchie himself if he wrote the code, as Bruce Perens suggested might be the case. His answer makes clear that this is definitely old code from the 70s, and here's what he says about authorship:
"So: either Ken or I wrote it originally. I know that the comments that first appeared by the 6th edition were definitely written by me, since I spent some time annotating the almost comment-free earlier editions."
So that's one piece we have nailed down. SCO said that they ought to know what they own. Well, I'd say Dennis Ritchie can be relied on to know what he and Ken Thompson wrote and when it was written.