The US Marine Corps UH-1N has a whole suite of avionics changes from the H model that many times alter the appearance of the airframe. (Besides, obviously, having two engines) The Marine UH-1Ns are equipped with a FLIR system, which consists of both a FLIR turret and a box-shaped bulge under the nose. The nose is also slightly pointy-er on an N. APR-39 and AAR-47 radar detecting antennas are installed two-each in two bulges on the nose, and two on the aft tailboom. There is also an ALQ-144 "disco ball" mounted above the engines, and a SATCOM antenna on the roof instead of the generic radio antenna. Also on the roof are two "bat ears" above each main cabin door for mounting laser detectors. There are also ALE-39 dispensers sunk into the forward part of the tailboom. And depending on if you are looking at a bird with SCAS installed, it may or may not have a stab bar on the rotor mast. Finally, there would be a significant difference in both the instrument panel and center consoles between the H and N models.