Might be Nagra pilot tone. Around 1kHz 40dB down. John Hall I knew your father, played Binaural tape thru headphones, and he was amazed. We ( Livingston Electronics ) made several recordings in 54 and 55. The studio was a large room. About 75x 50 ft. His hearing was down 8dB at 10kHz. Explaines the hot high end on the Mercury records. I have several of them. 615-859-3681