The WE13C transmitter and WE20B receiver weren't left behind, so what marine frequency radio did Allen and AE refer to?
She was referring to the Bendix RA-1 receiver that was installed with the Bendix RDF loop system just prior to the first world flight attempt in March.
The airplane was delivered in July 1936 with a WE13C transmitter and a WE20B receiver. The transmitter had two crystal-controlled frequencies, 3105 and 6210 kHs – standard frequencies for U.S. aircraft. The receiver could receive on four bands, 200-400 kHz; 550-1500 kHz; 1500-4000 kHz; and 4000-10000 kHz. It could NOT receive on 500 kHz.
When delivered, the aircraft had no homing device.
By September 3, the day before the Bendix Trophy race, arrangements had been made for the installation of “a new radio homing device recently perfected by the Bendix aviation interests.” (Source: Arcadia CA Tribune 9/3/36.) The Hooven/Bendix Radio Compass was installed while the aircraft was at Purdue for the month of October.
While the aircraft was on the east coast in February 1937, W.C. Tinus of Bell Labs convinced Earhart that she needed to be able to communicate with ships at sea via the universal 500 KHz calling frequency. That frequency was code-only but AE had recently brought Harry Manning on board and he was adept with code, so no problem. Tinus added a 500 kHz crystal to the WE13C transmitter.
At the same time, Earhart agreed to let Bendix replace the Radio Compass with a Navy RDF loop antenna system that Bendix wanted to introduce for commercial use. It was more complicated to use than the Radio Compass but DF would be Manning's responsibility so, again, no problem. To give the aircraft the capability to receive on 500 kHz, Bendix would install an RA-1 receiver with the new loop system after the aircraft returned to Burbank. The Radio Compass was removed and the RDF and RA-1 were installed in early March.
When Manning bailed after the wreck in Hawaii, communication on code-only 500 kHz was no longer an option so, to save weight, the RA-1 "marine frequency" receiver was removed and the Bendix RDF system was connected to the WE20B.