In the 1950s and 60s rock music, which could be heard on foreign radio broadcasts, was banned in the Soviet Union, but still very popular. Among Russian youth there was a black market for rock recordings. It took a bit of creativity to make these underground recordings, and one of the most common types was known as 'Records on Ribs'.
Old x-rays were scavenged. Because of the thick plastic they were filmed on, they could be cut into disks and -- using booths soldiers used to record cheap records for their relatives and girlfriends -- bootleg tapes could be recorded onto the used x-ray films.
They had the added advantage of being flexible, so black marketeers could wrap them around their arms and hide them under coat sleeves.
I tried to find an audio recoding of one of these old records, but had not luck. I found the three pictures in this post showing old Records on Ribs on the website Street Use, which includes more information about the practice and has some great comments in their comments section.