This shot of one of the filming models shows a door at the rear for the strut between the top two nacelles that I assumed was for a probe\torpedo launcher. Am I wrong about that?
Those are actually the bottom nacelles... The ship is mounted upside down in that picture!
Okay, that could be a door covering a torpedo tube - especially when the torpedo decks on the pylon junctures also have doors (or, in reality, don't have the muzzle bits glued on). Or, if we want to get fancy, it could be a door for a trailing sensor array...