IRC , it was the removal of the aux motor for the turret ( no small exhaust ) , and the longer L48 gun that make it a J model . The removal of vision / pistol ports , the dual exhaust , one piece commanders hatch , etc were all design changes to save assembly time and money witch started before the J came out . You see some of these changes on Gs & Hs . As well some Js still had ports , split commanders hatch , even old large exhaust (but no small ) , really a mishmash of parts which goes along with the German doctrine of using up old parts on newer models of vehicles . Maybe Rubicon could give us 2 set of turret doors ,one with , one without ports . The ports on the turret side and rear could be easily removed by the modeller if so inclined