"Roentgenizdat" or "X-Ray Pressed" records are 7-inch bootlegs made from discarded X-Ray plates found in hospital skips. They originated in the 1950s within the underground nightspots of the USSR and Eastern Europe, and contained pirated music from the West.

Information etc.:

http://www.kk.org/streetuse/archives/2006/08/jazz_on_bones_xray_sound_recor_1.php

http://bujhm.livejournal.com/381660.html

Roentgenizdat is the practice of imprinting an audio track on to discarded x-ray plates to then be played on a record player or gramophone, developed by underground music pirates in the Soviet Union during the 1950s until it was made illegal by the Kremlin in 1959. The name roentgenizdat comes from the combination of roentgen ray (another word for X-ray) and samizdat ("self-published", or underground literature). As well as literature, much western music (including rock and jazz, etc.) was banned... X-ray records were of poor quality and seldom lasted for more than a few months, but they still contained the precious forbidden music, and as such were treasured by all who could get their hands on them - Dave Loder