I cannot speak for all cases, certainly, but I understand that the
aircraft factories did the original installations of radio equipment
during the final phases of airframe construction, to include the cable
runs and antennas. I have some drawing copies from Consolidated that
show the placement of the BC-348, BC-375, direction finder (MN-26?),
interphone, control boxes, etc., and to include the associated cable
requirements and antennas within B-24 aircraft that they were
manufacturing, at the time. Also, when the aircraft were ferried from
the various factory locations to the designated military airdrome, the
radio equipment was operational. That is not to say that equipment was
not installed by the military.. to the contrary, especially when things
were upgraded, etc. As an aside, I also have a technical tome that
seems to indicate that Norden bombsights (the sight unit itself) were
installed by the military under some sort of an umbrella of security.

As a collector and user of a BC-348N and an R, used with an ATC-1 and a
BC-375, I appreciate the history of these receivers and would be curious
about the actual lineage of my units. I know one of mine was apparently
never put into service by the AAF. Rather, it was sold at surplus, NIB,
by Radio Shack in 1948. I have the original receipt, found inside the
cabinet, as bought from the original owner's family in 1999. My other
unit was obviously "used". I really don't have a clue as to how many
truly "combat-operational" sets made it to the surplus market. I
suspect most were decommissioned along with the associated airframes. I
think this is why there are so few rack mounts for many aircraft sets
vs. the number of radios. Try and find a rack for an ART-13! It took
me 15 years to find the actual aircraft rack (not to be confused with
TCZ ground version mounting).

Finally, I'll add a tech suggestion. Some BC-348 versions have a
gain-leveling pot, mechanically attached to one end of the tuning
capacitor shaft. Its supposed to keep the receiver gain level over the
tuning range of each band. You can get just a bit more gain by
bypassing this pot. If the pot is going bad, it can help a lot to
bypass it, of course!