The RIAA compensation curve is inserted at the lathe, and is not on the master tape itself.
Actually what it usually was, was Dolby NR un - decoded tapes. Dolby encoding was usually done at the end of the final mix to two track. But a lot of times, Dolby was applied at the first stage of the mix down to two track - IOW in say a 24T, there would be 24 individual Dolby units encoding each track. The 2 track, and the multi track tape boxes and track sheets were noted "with Dolby" (and which flavor, usually A or C), so whoever pulled them knew to fire up the Dolby rack. But in the rush to convert from tape to CD, as you say, a lot were converted with the Dolby encoding "un-Dolbyed" . And a lot were transferred without even taking the time to properly bias, or even align the the playback machine, so there would be this mis alignment / mis bias smearing, best heard at the top end, in the CDs. When CDs came out, record companies saw a goldmine, and they rushed transfers, often hiring people with not even a passing knowledge of audio engineering, to do they jobs-they hired the cheapest, and got cheap results. They would put these untrained people in a room with a 2 track and a CD recorder, and a stack of tapes. The idiots would clean the tape heads, open the box drop the tape on, set the levels, and hit play and record, then read a book for maybe 20 to 40 minutes, then pull out the CD ,label it, hit rewind on the tape machine, rebox the tape and go to the next one. And quite a few didn't even bother to set the levels, either. The Engineers and Producers that spent long hours getting the sound they wanted on the master tapes, saw their work ruined, and they were quite pissed when they heard what happened to their mixes.