I noted the open question about whether a separate direction finding (DF) receiver was installed; if so likely the Bendix RA-1. I did a web search on RA-1 equipment and was intrigued with what I found.
I'm about as far as one can get from being a radio expert, but I
think the current consensus is that there was
not a complete separate receiver for DF installed; the DF function was accomplished via a DF Loop Coupler as shown in
this post and others near it in that thread. The Loop Coupler connected the loop antenna to the main–and only–receiver, the Western Electric 20B. The Coupler was an experimental unit produced in the Radio Research labs in Washington during the short time after that company's acquisition by Bendix and before manufacturing operations were moved to Baltimore. Because experimental, no company literature or photos are known to survive (people
have looked). Its location in the Electra during the final flight is not a certainty, but there are photos showing it mounted back in the main cabin, on the starboard side, on top of the tall fuel tank. Others should jump in and correct the above if I have mis-stated something.