I googled Live Performance, La Boheme February 16, 1974 and came up with a live performance with Caballe and Corelli at the Met (Metropolitan Opera in NYC). Available as a 2 CD album on Amazon. It could well have been recorded by NBC. However, the version on Amazon appears to be a pirate. Caballe and Corelli were two of the best singers of that time period (Freni and Pavarotti are maybe the best and in the Decca/London studio recording with Karajan conducting about the same time - I saw them live in 1967 at the SF Opera in Pavarotti's official American debut.) Anyway, if it has good sound, it should be a real treat.
What is the recording - hopefully 15ips 2 track? Should be about 90 minutes long or a bit more if it is complete.
Larry