Unreleased Led Zeppelin tapes for sale - auction details here and here.
Ron Nevison, the engineer on the classic Led Zeppelin album Physical Graffiti, is selling tapes made at the sessions, which, according to the labels, includes early takes of In My Time of Dying, Trampled Underfoot, Sick Again, The Wanton Song and the rare Swan Song.
I have a couple of bootlegs of these sessions, but I'm hearing that these tapes may contain versions not released on the boots (Physically Present and Totally Tangible).
As I understand it, a producer owns the tapes he's made, and can sell them - but that doesn't mean he owns the copyright, or can sell copies. (As you can see, the copyright holders have already had the videos on the auction site taken down.) So unless these tapes go to Jimmy Page, which is always possible, the tracks are unlikely to get a wider release.
Old tapes are often playable - once. The tape reels are baked slowly in an oven for several hours or even days to dry out all moisture. They can then can be remounted and played back. They are usually copied to modern media at this point, as the old tapes will continue to stretch and stick due to age.