Typically the Japanese Reissues have Basswood bodies, though I think they changed to Alder for some variants sometime in the 2000s, and for the Japanese market I have seen some Sen Ash examples in Japanese stores. I can attest that Basswood was used in the 90's and early 2000's as those guitars were all pretty light and the few I've seen with dings open enough to see the wood beneath have that telltale salmon tint basswood has under the sanding sealer.
I was having a small debate about this with someone on OSG a few weeks ago with regards to the Jaguar/Jazzmaster and there was a conclusion that Fender Japan uses different woods for different markets. Basswood is a pretty inexpensive tone wood, and most Japanese building houses use it for their "shred guitar" builds. Japan would have some more expensive tone woods on their own market of the Mustang because the Mustang is a much more popular guitar there, and Japanese guitar players probably demand more options for the Mustang than we do in the states.