One of my absolute favourite prog rock bands, in the 70s and still now, is Gracious! You may not have heard of them. They did support The Who, King Crimson and others for a time.
They were on the famous Vertigo label.
Their first released album (1970) has the famous white cover that has a textured effect and gathers marks like you can’t believe. I remember buying mine in Bernard Dean’s in Scarborough and being given a clear protective sleeve because the cover had picked up marks on the rack. It cleaned easily enough with a rubber. I still play it.
It’s a concept album of sorts. Side one consists of a self-referential Introduction, Heaven and Hell, all linked with odd beeping noises. The second side has a fugue for harpsichord and guitars, then a lengthy piece with bits of Beethoven included. There’s lots of mellotron, of course.