Bob,

Your adapter panel is the MX-19/ARC-5 ILS Audio Adapter that is designed **solely** to connect an R-23 or R-24/ARC-5 receiver to serve as the MF LOCALIZER and MARKER BEACON receiver for the USN's AN/ARN-9 Air Track ILS.  That ILS was abandoned by the USN in 1944 in favor of the USAAF's far superior SCS-51 ILS that is still used today.

The MX-19/ARC-5 is not especially rare...but it is one of the very few surviving remnants of the AN/ARN-9 ILS.  Very few know what it was for.  FWIW, below are a few optional details that won't be interesting to many.

Mike / KK5F

Optional Info

A.  USN AIR TRACK ILS

AIR TRACK ILS was developed around 1935 for civil and USN land base use. It was installed at some bases for patrol aircraft. The system consisted of:

MF LOCALIZER 300-800 kHz, lobes modulated at 90 and 150 Hz.
VHF GLIDE SLOPE 90-98 MHz, lobe modulated at 60 Hz.
MF MARKER BEACON 300-800 kHz, uses different transmitter on same frequency as LOCALIZER, modulated with an unspecified audible tone.

Ground Transmitters
USN Type YB, YC, YD (1940)
YB-* Includes YC-* plus YD-*.
YC-* LOCALIZER and GLIDE SLOPE
YD-* MARKER BEACON

Aircraft Receivers
Early
USN Type ZA, ZA-1 (1940)
ZA-* GLIDE SLOPE; LOCALIZER converter.
RU-* LOCALIZER and MARKER BEACON
 
Late
AN/ARN-9 (Improved ZA)(1943)
R-43/ARN-9 GLIDE SLOPE; LOCALIZER converter
R-23 or R-24/ARC-5 with MX-19/ARC-5 audio adapter for
LOCALIZER and MARKER BEACON.

B.  USAAF SCS-51 ILS

SCS-51 ILS was developed around 1942. The system consisted of:

VHF LOCALIZER 108-110 MHz, lobes modulated at 90 and 150 Hz.
UHF GLIDE SLOPE 332-335 MHz lobes modulated at 90 and 150 Hz.
VHF MARKER BEACON 75 MHz, modulated at 1300 Hz.

Ground Transmitters (typical)(1943)
AN/MRN-1 LOCALIZER
AN/CRN-2 GLIDE SLOPE
AN/MRN-3 MARKER BEACON

Aircraft Receivers (typical) (1943)
RC-103-A LOCALIZER (BC-733-*, 6 channels)
AN/ARN-5 GLIDE SLOPE (R-89/ARN-5, 3 channels)
RC-193-A MARKER BEACON (BC-1333).

In 1944 the USN also adopted the SCS-51 ILS. By 1948 this system had been adopted worldwide as the standard military and civil aviation ILS, where it remains today with the addition of more LOCALIZER and GLIDE SLOPE frequencies, but otherwise identical.

C.  AN/ARN-9 ILS AND AN/ARC-5

The AN/ARC-5 Maintenance Handbook refers in several places to the use of the R-23 and R-24/ARC-5 navigation receivers as part of an unspecified ILS.  That ILS was the AN/ARN-9.

The AN/ARN-9 consisted of:

R-43/ARN-9 Glide Slope receiver and Localizer audio converter
C-62/ARN-9 Control Box
ID-24/ARN-9 Cross-Point Indicator
J-32/ARN-9 Junction Box

AN/ARN-9 required LOCALIZER signals to be provided by the AN/ARC-5:

R-23 or R-24/ARC-5 Localizer and Marker Beacon receiver
MX-19/ARC-5 Audio Adapter

R-23, R-23A, R-24, and R-148 navigation receivers are the only AN/ARC-5 receivers that provide an audio signal to the adapter panel connector J-1.  However, it does not come from the receiver's AF output transformer.  It is low-level AF developed across the 12A6 cathode resistor.  It is filtered for the 150 Hz AF signal used by AN/ARN-9.

The sole purpose of MX-19/ARC-5 is to connect the receiver as Localizer to AN/ARN-9.  In addition to audio, the MX-19 contains a relay controlled by AN/ARN-9 that shuts off the CW oscillator and sets maximum RF gain when AN/ARN-9 is in use.

Components and documents of the abandoned AN/ARN-9 Air Track ILS are essentially non-existent today except for the ID-24/ARN-9 cross-point indicator and the MX-19/ARC-5 ILS Audio Adapter.