I'm not sure if model 300 has the same characteristics but this is what Jay McKnight said about the 350 recently on the Ampex list (if it's not kosher to quote from the Ampex list, someone please let me know and I'll erase this):
"The 350 repro head cable has two independent shields, and the
inner one is "driven" by the repro pre-amp in order to reduce the
effective capacitance between the signal lead and ground. The outer
one is grounded.
There's nothing special about the rec & erase head cables."