I haven't compared the two recordings, maybe I did some time ago, but I don't remember. I'm sure the Classic Recording was new when I got it and I did buy both the 33 and the 45 versions. The later version I definitely bought used, and I don't remember the condition. Both are gone as well as the rest of the vinyl collection. I did buy a tape (15ips 2 track) of the early version, which is a dub from a production or safety master from my Canadian source of classical tapes. That sounds very fine.
I think RCA generally did an excellent job in their early stereo recordings and their early tapes. I have a fair number (maybe 40+ reels) of early RCA 7.5ips 2 track tapes commercially released in the '50s, many before the stereo LP versions were released. One of them, the Gaite Parisienne by Fiedler and Boston Pops has the price printed on the front cover, $14.95, which was at the time around four times the price of the mono LP. It is stereo as are almost all of the RCA 2 track tapes I have. So then as now, buying precorded tapes (particularly 2 track tapes) are not for everyone. Lot more bang for the buck buying LPs.
In the early days, the mic number and placements were much simpler, so mixing was easier, and also not as much to mess up.
The GCS-6 designation is the catalogue number of the 7.5ips 2 track tape, recorded in November, 1954 and released in April 1956, the same time as the mono LP, LM-1900. This info is from Discogs. Bertelsmann had no relationship with RCA back in those days. So I don't know why BMG should be listed with the tape catalogue number.
Just to be clear, RCA itself never released the Berlioz as LSC-1900, but did release the album in the 1980's in a low price reissue series as AGL1-2706 and then again as AGL1-5203. When Hobson released his Classic Records reissues of the Living Stereo, he included several titles that were never issued by RCA in stereo. These included both the Berlioz and the Brahms Piano Concerto as well as several Decca engineered albums commissioned by RCA, which never were issued in the US in their Living Stereo series. I found the catalogue detail on the first line in Discogs also.
Larry