Fifty years ago was an auspicious time in popular music.

In 1964, the Beatles appeared on Billboard’s charts for the first time, the Rolling Stones released their first album, the Supremes had five No. 1 hits and Simon and Garfunkel debuted with “Wednesday Morning, 3 A.M.”

The 50-year milestone is significant, because music published within the first half-century of its recording gets another 20 years of copyright protection under changes in European law.

So every year since 2012, studios go through their tape vaults to find unpublished music to get it on the market before the deadline.

That’s how we came to get outtakes of Bob Dylan, the Beatles and the Beach Boys in recent years.

This year, Sony is releasing a limited-edition nine-LP set of 1964 recordings by Mr. Dylan, including a 46-second try at “Mr. Tambourine Man,” which he would not complete until 1965.

The Beach Boys released two copyright-extension sets of outtakes last week.

There’s no official word on a Beatles release, but last year around this time, “The Beatles Bootleg Recordings 1963” turned up unannounced on iTunes.