Awhile back when I was thinking about building a Jaguar I was looking into compatibility between necks and bodies. I remember reading someone saying that the Japanese and American necks don't always fit snug in each other's bodies. I forget which was which, if the American necks had a gap in Japanese bodies or the opposite. Anyway, I had a Squier Jaguar awhile back and decided to try this out and put it up here for reference for anyone else.
Here's my 90's CIJ Jaguar (got in Japan, not sure if that makes a difference) and a 2014 Squier HH Jaguar. Also not sure if MIJ or more recent Japanese Jaguars would be different.
It looked like the Squier neck fit pretty snug in the Japanese body except for a small space near the heel. The Japanese neck left a small gap in the Squier body near the heel and near the back of the neck. The gaps were pretty minuscule, but I guess it could bother some who are particular on that sort of thing. Also worth mentioning is the screw depth. The screws for the neck plate were a little different, but I just used the screws that went with the body, so no problem there really.
Anyway, I hope this helps someone in the future. I'm under the impression that USA and Squier parts are pretty interchangeable, so maybe my findings would apply to USA and Japanese parts too. Unfortunately I didn't measure the heels on each and no longer have the Squier. Next time I take apart my CIJ Jaguar though maybe I'll measure the neck heel.