The three photos (on the previous page of this thread) show a number of differences.
Photo 1: Lead-in for dorsal antenna is high in the fuselage cabin. In this photo we can see the first stand-off support for the lead in, but it may be a different style standoff from what is in Photo 2. For the antenna, the stand-off supports would have been important as the antenna lead-in has no shielding, and you don't want close nearby metal to affect your antenna coupling and you want to control where the lead-in wire is so it doesn't move about and affect the pre-fixed tuning of the transmitter (i.e. you want to control the parasitic capacitance to ground). Transmitter tuning did not involve maximum signal into the antenna, but minimum signal reflected back into the transmitter. Too much signal reflected back into the transmitter results in heating of the output power tube.
Photo 2: We do not see the lead-in entry. We do see the lead-in as it goes vertical on the sidewall. The lead in is supported by ceramic standoffs (a proper installation practice). In photo 2 we do not see the ends of the 1"x 1" wood supports for the fuel tank crawl-over platform (that should be near the arrow of C and visible in photo 3). So photo 2 may be prior to a crawl over being installed. Does the 'pipe?' in C represent a different fuel-tank vent setup? (I assume those are the tank vent tubes) Feature A on the WE 13C Transmitter is definitely an antenna ground connection. It can be seen in this photo of another WE 13C.
https://kn4r.com/kn4r/Western_Electric_13-C.html And the ground connection for the antennas shows up on the schematics.
https://aafradio.org/docs/Western_Electric_WE_13C_Transmitter.pdfPhoto 3: With D it appears that a wire harness is now running along the fuel vent manifold, and tied-down with a wax lacing cord?/wire loom method. As the cable comes to the end of the vent manifold, it goes down the cabin wall - indicated with arrow E. but at E an additional black cable can be seen. Is this the antenna lead-in? That would explain the shielding added to the cable. Shielding the harness would be necessary to keep antenna RF from coupling into the wiring and creating issues elsewhere in the aircraft. Also, the point at where the lead-in exits through the cabin wall cannot be as high as it was in Photo 1, so Photo 1 and photo 3 must have had different antenna arrangements.
Frankly, if I have interpreted photo 3 correctly, the lead-in laced directly to a shielded harness looks to be poor installation practice for this kind of open-wire line. I'm not familiar with acceptable signal and transmission line integrity practices of the 1930's. Close proximity results in a some of your transmitter power shunted somewhere other than the antenna. No doubt that when they did this Photo 3 installation they re-tuned the transmitter output, but that only keeps the transmitter happy.
Since installation manual for a WE 13C Transmitter is not available, I went a aircraft transmitter with similar power output, frequency range, and antenna attachment method. The following is from a installation manual for a military ARC-5 "Command Set" transmitter developed in 1936 and used throughout WWII until the early 1960's. Document: "AN 16-30ARC5-2" Dated Dec 15 1954, Page 9 section 2-6 "Installation" paragraph i. followed by paragraph l.
i. Short antenna leads inside the fuselage are essential. This requires the use of the minimum, practical spacing between the antenna relay unit, the antenna binding posts on the receivers and transmitters, and the antenna lead-in insulator. Bare wire supported on ceramic insulators, where necessary, should be used for all antenna connections. These installations precautions are necessary to minimize voltage breakdown, to reduce r-f losses, and to keep to a low value the capacitance to ground of the antenna wiring inside the fuselage.
l. To accomplish proper grounding of the transmitters, connect a 'short' flexible lead from the aircraft frame to each of the ground binding posts on the rack. ...... Reduced antenna current from the transmitters will result if these precautions are not observed.
You indicate the NTSB lab reported a wire diameter of 0.024". If a precise measurement (neglectable diameter change from corrosion), then this is the exact nominal diameter of 22.5 AWG wire (0.0240"). This is also 0.61mm. Solid magnet wire is available in 1/2 gauge steps in the present era but what about wire in the 1930's?
I used to play with some of the radios back in the 70's when a teenager and actively experimenting amateur radio ham. Today I work as an electrical engineer (designing power inverters for the aviation General Aviation market) but have done a bit of RF work in my career.