Tapes from Led Zeppelin sessions for 1975’s Physical Graffiti will be auctioned at next month’s Marvels of Modern Music auction.

Rolling Stone reports that Amherst, New Hampshire–based RR Auctions says many of the songs on the tapes differ structurally from the ones that are on the album.

“The band recorded the songs at the Ronnie Lane’s Mobile Studio, which audio engineer Ron Nevison built in a 26-foot Airstream trailer,” Rolling Stone reports. “The tapes are part of the Ron Nevison collection, which is also auctioning rough mixes of Bad Company’s debut and four songs from Eric Clapton’s 1973 album Eric Clapton’s Rainbow Concert, as well as recordings by the Who, Ozzy Osbourne and Flo and Eddie.”

Listen to brief excerpts of the music:

Fragments of “Kashmir” and “Custard Pie”:

“In the Light”:

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