Could you take a picture with a ruler for scale? Audio cables will be smaller than power cables and I recall examples of both that look very similar to that cable. If there is any brand name or part number on the cable, that would aid in identification. I think there is something stamped on the cable but rotated 90 degrees from the camera.
...Not sure what a 150' diode socket is.
The following is taken from a manual for Phillips LBB-1143
For connection to a tape recorder having a 150‘ DIN diode socket, tape recorder accessories cable EL 3768/10 may also be used