70 years after the death of the composer
50 years after the recording was recorded.

Do you have specific recordings in mind? I'd say your best bet is to trawl through CD labels that release archive radio broadcasts, a lot of their releases were recorded in the 1950s. Try the Medici Arts and Testament CD labels, they remaster and release radio broadcast performances on CD.
(The Empress it seems you CAN polish a turd!, Tue 9 Jun 2009, 14:50, archived)