Hi, guys,
My guess is it reads 海鷲 or umiwashi, meaning "sea eagles", as were called Japanese fighters (and pilots) during WWII. The explanation is as follows:
1) Till the end of WWII, when Japanese was written horizontally (as were magazine titles, shop signs, company names on trucks, etc. ) it was usually written from right to left. The guy who wrote this must have remembered that.
2) 海鷲 was not the name of a specific model of Japanese fighters as 紫電改 etc., but general nickname for fighters and pilots (there are films entitled 桃太郎の海鷲, an anime, 海鷲, a documentary, etc. (Which means this sign is not a genuine one, because no fighter had this written on them or, for that matter, no fighters had their names written in kanji on them, if not model numbers. It's written on the replica just for a show.)
3) As for the kanji 鷲, I first suspected it was an 異体字 (variant) of 鷲, but a look up of my 漢和辞典 proved it wasn't (there's no 異体字 for 鷲). My guess is that might be because of it being written in stencil (it's hard to think a Japanese military fan who wrote it made such an elementary mistake about such a rather popular kanji like 鷲 when cutting out a stencil plate or painting otherwise. Why stencil? You see the bottom stroke of the さんずい of 海 is divided in two. And the look of the photograph suggest that the letters were first stenciled and then retouched. When writing letters in stencil, they often change strokes not to let parts drop off...., but for this point I'm not really sure.
taktak