The story has always been that the 6 extra tunes were lost. The titles did not come out of the blue. The sheet music for a couple of these just recently turned up. Is it possible they never actually got recorded at the session and were just on a list - yes.
If they had been found and slapped on to a reissue, I hope the word would have gotten out.
This does seem to be a generic description.
I have seen Japanese reissues that do not list the bonus tracks on the cover, e.g. Duke Pearson's Angel Eyes.
I saw the Nichols CD in France, considered getting an upgrade even though no bonus tracks were listed, and decided against it. My experience with the Pearson had made me wonder. Then I saw this listing yesterday which made me wonder even more.
Ideally, someone on the board would have bought this, but apparently not.