Relatives going through the belongings of the late British singer Cilla Black inadvertently discovered a treasured piece of Beatles history: Paul McCartney's long-lost demo recording of 1964's "It's for You," a song the Beatles gave to Black.

Now, some lucky fan gets to own it. You can listen to a 20-second snippet at the Beatles Auction website.

The disc was found inside a brown envelope with the title of the song and Black's name. Her relatives assumed they'd simply found a copy of her single, and brought it along with other recovered items to be appraised at Liverpool's Beatles Shop. There, longtime manager Stephen Bailey went through 21 other discs before realizing what they'd found.

"We got to the last one, and as soon as I heard it, I thought, 'Oh God, that's not Cilla Black. It's Paul McCartney,' Bailey told the Telegraph. "I was shaking with excitement and speechless. I realized that this was the long lost Beatles demo disc from 1964, and I was probably one of the few people to have seen and heard it in over 50 years."

Perhaps predictably, the recording is in near-pristine shape. "Apart from a few crackles, which you get with acetates, the quality is fine," Bailey added. "It's a wonderful recording."

Black, who died last year, took "It's for You" to No. 7 in the U.K. in the summer of 1964. George Martin produced the song, and McCartney played piano. The Beatles' original recording is expected to be sold for between $20,000 and $26,000 when it's put up for auction on Aug. 27 at the Liverpool Institute for Performing Arts.

The Top 100 Albums of the '60s

More From Ultimate Classic Rock