Charles, I think it means that the original EQ (IEC I assume in this case) was preserved without any additional EQ being applied to the tape. I do have a few tapes which say "Production Master" with a notation that they were equalized, limited, etc. I assume for cutting vinyl. When I listened to the tape, with IEC, I think it sounded right - it is a rock album, so I don't have a good reference, certainly not in my collection. :-)
Larry