I'm generally not interested in 'historical' Mahler recordings, but . .

[barryguerrero]

Archiphon is going to issue a CD of a recently discovered ORF broadcast tape from the 1947 Salzburg Festival. On it Otto Klemperer conducts the Vienna Phil. in Mahler 4 with Hilde Guden, and, oddly enough, the 3rd Symphony of Roy Harris! The sound may not be so great, but it could be interesting to hear Klemperer doing M4 almost 20 years before his EMI studio recording (interesting performance, but Schwarzkopf does nothing for me).

https://www.jpc.de/jpcng/classic/detail/-/art/henry-purcell-otto-klemperer-live-in-salzburg-1947/hnum/11066595