I Googled "japanese zero-sen airspeed indicator" and noticed it pegs out at 400, so that would be mph, as kph would be too slow. Or maybe I'm not reading it right. The numbers look like they are x10, thus 38 is 380 mph.
I had read somewhere that the Zero's ailerons would become increasingly heavy at speeds above 300 mph, to the point where the pilot would have difficulty moving the stick.