Pat -- I had the same reaction to "Henderson Field" as you did -- but, a quick Google search shows that it's legit...
http://www.airnav.com/airport/PMDY
FlyingFort -- the B-25 (PBJ?) idea shows promise.
Don -- the PV-2 looks like a great possibility as well and certainly makes sense in context. If the good Col can check his records for a likely candidate, I bet Craig at AAIR (not to mention the Midway airport manager) would be appreciative.
I wonder -- were the top turrets for the Mitchell and Harpoon made by the same manufacturer? (were they a pretty standardized thing?) And how might you distiguish between the mounts for them on two fairly similar sized aircraft? There's probably some clues in the internal structure, but that's way beyond my pay grade. I imagine there are WIXers who work to maintain and/or restore one or both of these types that might be able to shed some light on the subject.