In the 1920s and 1930s there was no tape, so studios cut recordings directly onto wax discs. Because a lot could go wrong, they played safe by simultaneously cutting two discs. Sometimes they played extra safe by using two microphones, one for each disc. The result was a matched pair of recordings, each with a different sound perspective.
Brad Kay hunted down matched pairs of old discs and tried playing one as the left channel and the other as the right. Some engineers who heard his ‘accidental stereo’ recreations thought it was just an illusion created by slight playback differences between two identical recordings. Others thought the stereo sounded too real to be written off.” (New Scientist)
A new CD release of Edward Elgar’s music conducted by the composer includes a 1933 overture in “accidental stereo”.