Let me throw out another idea for consideration.
Ric's diagram of the dash control panel has a switch marked "Voice/CW", obviously used to select between a microphone or a telegraphers key as an input to the transmitter. As I understand it, wiring for a key would be less complicated than for a microphone.
We know AE/FN were not proficient in Morse Code. The dash control panel was likely made with a CW option for use by someone, presumably Harry Manning, for the first attempt. I think the most likely place for the installation of the key would have been on the nav table in the back end of the aircraft as the key would need a firm, stable mounting to be used properly. I cannot be sure of that but putting it up front with no surface to rest it on seems unlikely. This option would also require another wire pair going back for a headset for whom ever was sending Morse Code. Maybe that was too complicated and the key was used in the right seat up front where a headset/earphones were available. (Did they even have/use a headset? I seem to recall a picture of AE holding something to her ear while sitting in the left seat.)
If there was never any intention for anyone in either attempt to use CW, that argues the case that the dash controls were "standard" for a Western Electric installation and they or their successor may have pictures/plans somewhere.
If installed at the nav table that means there was a long wire running from the transmitter behind the cockpit to the table. I'm guessing that after the ground loop in Hawaii it was never removed. That may be what was later scavenged by the colonists for use elsewhere.
Without looking for pictures of any period telegraphers keys, I seem to remember the connection at the key being stripped wires held on with knurled nuts on screw posts. Joe's connector could have been used at the transmitter end.
Just another idea. . . .
Further searching. . . Harney's drawing of the Electra interior shows the transmitter located nearly underneath the nav table and a "lead to transmitter" annotated. The longest wire would then be from the cockpit location of the of the control panel back to the transmitter. Harney's drawing doesn't show a telegraphers key, as such, but that "lead to transmitter" could be for the key.
Still looking for a photo of nav table prior to takeoff on first attempt to see if key and headphones are shown.