Different service, different designations.
uhmm, donīt mean to be picky, really... the pb4y-1 privateer is indeed based on the b24d/j/l/m model, but with substantially different avionics; manned with a operating crew of eleven; also a different front turret (first a-6a, later erco 250, those well known ball-like ones).
pb4y-2: a completely new air frame, inspired by the b24j/pb4y-1. all new rear section ('conventional' single tail fin & rudder); wing profile optimised for operation at low altitudes; also a new front turret (again erco 250 SH, can also be found on the later pb4y-1); waist gunnery replaced with erco 250 'tear-drop' mg nacelles; belly turret being now obsolete, therefore removed; p&w r-1830-94s, no longer equipped with superchargers, therefore redesigned engine nacelles; avionics specifically for naval operation/warfare; capable to fire wing-fitted asm-n-2 'bat' projectiles (radar-guided); and finally stretched (again) by 7 feet before wing sect to fit in an additional flight engineer.
a neat one :gg: you may call me smart-pants if you like to. :g: regards