Further on Columbia stereo ...
The IS Agon was recorded at Goldwyn Stage 7 in Hollywood, June 17, 1957.
From 1957 - 1959, Columbia often had both two- and three-track running at orchestral sessions.
If we really want to be complete ref. stereo, we ought to credit the Dutch / Philips-Miller experiments recorded onto film in 1939 - 1940. To my knowledge, these recordings have never been published.
On RRG - From the summary of Heinz H.K. Thiele's presentation on RRG stereo at AES in Berlin in 1993:
'Approximately 200 recordings, mainly of classical music, were made at the RRG. Only five of these recordings remain in existence today -- the others could not be found after World War II.'
The missing reels undoubtedly went to Moscow where they were degaussed and reused by the Russians on captured Magnetophone machines.