This was apparently just an unusual way of producing vinyl records (themselves a dead medium), only briefly described here. However, as this form of record reached a certain geography that was otherwise cut off, and since bone music had its own network of distribution and underground production, I think it's worth mention. The comment in parenthesis is Brodsky's.
"...in the Fifties every city youth had his own collection of so-called bone music. 'Bone music' was a sheet of X-ray film with a homemade copy of some jazz piece on it. The technology of the copying process was beyond my grasp, but I trust that it was a relatively simple procedure, since the supply was steady and the price reasonable.
"One could purchase this somewhat moribund-looking stuff (talk about the nuclear age!) in the same fashion as those sepia pictures of Western movie stars: in parks, in public toilets, in flea markets, in the then famous 'cocktail halls'..."
(((bruces adds: Artemy Troitsky's BACK IN THE USSR, a history of the Soviet pop underground, also describes the very extensive Soviet practice of creating and circulating illegal recordings on used X-ray plates.)))