Phonograph records didn't fall under copyright until 1971 (the ℗ symbol stands for Phonogram). But discographies will often list both the recording date and the original issue date. The biggest general online discography I know of is www.discogs.com; www.allmusic.com is another place. It's possible you'll be able to find specialized discographies, e.g. for the Boston Symphony Orchestra.
You also may find information about specific albums in Wikipedia, but it's hit or miss -- whether the album is even there, or if it is, how much detail is included
So far some of them have been easy, an old Pink Floyd and Mama Cass album.