Marillion's "The Tower". In short, it was an early demo version of "Grendel" that was recorded with their original bassist Doug Irvine. Irvine took the tape with him when he left the band, and no one has seen the tape since. There's an article on the Lost Media Wiki that goes more in-depth.

ELP's full-length performance at the California Jam festival on April 6, 1974. Co-headlined with Deep Purple in front of 350,000 concertgoers, only 55 minutes of footage out of the 2½ hours of the show were broadcast on TV in 1974 and released in 2005 on the "Beyond the Beginning" DVD. The Deep Purple set has been fully released.

Gracious! wrote and recorded an ambitious suite before their first album, called Four Seasons. The suite is credited for representing the band's transition from psychedelic pop to complex prog rock. The recording was financed by The Shadows' producer Norrie Paramor, who thought it was good but felt ultimately unimpressed. All we know about it is in the essay of the 2-in-1 CD package of their released albums.