OK, I'm back. The main thing is to figure out WHY a dupe tape would be marked MASTER and the original tape marked "DO NOT USE." That is key. Clues are listening, reading the old notes, talking with the people involved in the recording and mastering the original.. Fast example, Bell Sound, NYC used to recopy EVERY tape that came in that wasn't recorded at Bell originally. For the billing. So what they marked MASTER was actually a dupe. Drove me (and Bill Inglot, etc.) batty. The originals were maked simply "ORIGINAL." and in all cases sounded better than the master (obviously). Sadly, ABC Records razorbladed many of these originals back in the early 1970's to make vault space, because they weren't marked with the magic word "MASTER". Good for us that they didn't touch the Ray Charles stuff (Ray had a buy back contract) so the originals of all his ABC-Paramount stuff were used in my work. Other companies kept most everything. Still, an album like PETER, PAUL & MARY's first album has a three track for the songs, a mono original, stereo original, Radio Recorders "master" (a remix) and a bunch of safety dubs. It was a pain but our Audio Fidelity SACD was a mixture of a few remixes I did (for noise level lowering) and the original NYC stereo mix. Other albums took a long time to find originals of. Not what a mastering engineer is supposed to do, they use the music that is given to them and don't complain.