So, despite the lack of photos showing Pan Am Clippers with loop antennas, at least some Pacific Division ships were equipped with Bellini-Tosi DFs. As mentioned at the beginning of this thread, in the Google docs timeline for May 24-29, 1937, cited newspaper clippings describe PanAm technicians installing a radio direction finder in Earhart’s Electra "similar to those used in South American and Pacific flights.” Did Pan Am, in fact, install a Bellini-Tosi DF in NR16020? I don't think so. Discussions about doing so might have prompted the newspaper articles, but there was no visible change to the loop antenna on the aircraft. It's clearly a Bendix Mn-5 rotatable loop and the Bellini-Tosi DF is a fixed-loop system.
Incidentally, in researching this I stumbled upon an error in Finding Amelia. On page 74, in describing Earhart's radio tests in Lae, I wrote:
"It is not clear whether Balfour’s previous ground test of the receiver
included taking a bearing using the direction finder, but it is known that he
carried out his test on a signal of 500 kilocycles, a frequency well within the
loop antenna’s 200 to 1500 kilocycle capability."
It is NOT known that he carried out the ground test on 500 kilocycles. That comes from Elgen Long in his book "Amelia Earhart - the Mystery Solved" page 183. Long cites the
Chater letter but Chater makes no mention of the frequency used. In fact, a test on 500 kcs would be impossible because the Western Electric 20B receiver could not be tuned to that frequency (which was the reason for the whole abortive installation of a Bendix RA-1 receiver).
This is another example of Long stating his own assumptions as fact. My bad for falling for it.