Airfix didn't start producing kits until the mid 1950s so models would mostly have been scratch built or made of wood or die cast metal, model railway models would be the main subject. Also manufacturing was less refined than now. There is also the collectors issue which means anything this old, especially if never opened and built.
As to what came out in the 1950s, looking at the current Airfix catalogue the Gloster Meteor FR9 and F8 and Avro Shackleton MR1 (but not he AEW2 which is currently in the catalogue so you would need to find the earlier release) hit the 1950 date.
Near misses include the North American Sabre and MiG-15 which both debuted in 1949. The MiG 15 hasn't been rereleased but should be possible to find. The Jet Provost and Vampire T.11 are a little later.
Other manufacturers will almost certainly have other subjects which hit the date but I don't have the reference material to offer suggestions but you would be looking at the early jet era for aircraft of which there should be plenty of subjects, especially for US aircraft. I can't offer any advice on ships.