You say there was a third cotton cord. Was it a complete wire like the other two, or only a cotton cord? The looms had a third cord which had no wire inside, it was meant to be threaded into the earphone and knotted, and was there to prevent pulling on the headphone cable at the earphones.
The third string I found is indeed a wire-less twined cotton cord, knotted inside in order to prevent pulling on the cables, just as you noted and as I'd expect had it been a B/early C type helmet.
the helmet has a known history and has never been tampered with as it came directly from the airman's family.
The type served as a WOp/AG and after finishing the Signals Instructors Course he was a Signals Leader with No
311 (Czech) Sqn. He retired from RAF in January 1946.