I can't check much at home but I can try to answer : this 'opening and modernization' is mostly a period of accelerated capital accumulation, with incredible poverty and of course rural exodus. Plus the search for political freedom and the new possibility to leave the country (after 250 years of stricly enforced closed country policy). I know there are stats about the kind of people who left (business men, workers, peasants...) but I don't have them here. :-(
Why SA ? Well, obviously not only SA. Also Hawai, Hokkaido (the big island in the north, that is part of Japan only after Meiji) and also Korea, China when military imperialism starts to take over, also the USA. I mentioned SA only because it is mostly the descendants of emigrants to SA that came back through the new visa law. The ones in the USA fared much better, and the ones in Korea and China either came back right after the war or were kept (in China) until very recently (but their children have easy access to Japan soil through other kinds of visas). I suppose all the figures are somewhere but we have a long vacation (three days ... they call it the Golden Week) so I won't be in the office for a little while. I'll try to do better next week :-)
JC Helary