One of the reasons I picked it was because, by the time the album was released, the Beatles had effectively broken up. There's not even any clear evidence that the all four Beatles were ever in the same room together after September 1969. So unless Apple or EMI had made arrangements for the Beatles to sign some promo copies, an autograph collector would have had to go around and have each of the Beatles sign individually. Which could be done, but it would have required a lot of effort - especially after Lennon moved to New York. But sure, I take your point.
This would make "Let It Be" rarer, wouldn't it? Band officially kaput when it was released. I'd bet that pretty much every "all four signed" albums after the Beatles left the road came from individual signings, not grabbing all 4 Fabs at once. They just weren't regularly accessible to the public all at once much after 1966.
According to this article, only 2 fully signed copies of The Beatles and only 2 fully signed copies of Abbey Road are known to exist. It also says that there are no known fully signed copies of Let It Be. It goes on to say only 12 US album are known to exist fully signed. The White Album Project | A Comprehensive Look At The Beatles Self Titled Double Album Masterpiece
Don't know why but first thought for rare was Johnny Ace. Robert Johnson wins though! Ritchie Valens was around longer than two months where people would've asked for autographs, closer to half a year.
rare ? I saw a Brian Jones autograph in the 80's or 90's and it was priced quite high as I recall ... can't imagine it is too easy to come by these days