Well .... have they searched in Japan? As I mentioned above, I remember there were some Japanese CD reissues, so maybe .....
FWIW, here's the listing and commentary from the Felsted Discography page:
This doesn't even mention the Japanese issues.
I'm spinning the Japanese Polydor P33L 20043 CD issue of the Strayhorn right now - sound is okay, although the volume level is somewhat low. There must be some tapes - check Universal Japan, Affinity, Master Jazz (Mosaic did a Master Jazz box set so they should know where to ask). If that doesn't lead anywhere, I'd loan my CD copy any time - for a copy of the box set, that is ....