I've enjoyed these pieces of Sekai info that have floated in the past
few days. Last year I offered on-list a Sekai 4000 frameset (which we
still cellar, btw... ;-) ), and struck up a discussion with Mr. Black
when he noted his Centurion Semi-Pro was near-identical (save for the
Shimano dropouts on mine)... here's my 1976 Sekai 4000 (57x57),
"M6xxxx" serial. Prior owner had also indicated this was Champion #1:
http://www.flickr.com/photos/joe_bunik/sets/72157625729982233/
I'd read somewhere more regarding the Miki(/Miko???) and Yamaguchi
connection, but can't recover them! Was Yamaguchi employed by that
shop?
This thread, too has some possibly relevant trivia:
http://www.bikeforums.net/showthread.php/429693-Sakai-Bikes
... which corroborates a mysterious comment Mr. Black once made to me,
"About ten years ago I had a KHS touring bike from the early 80s, that
had a seattube sticker that said, Parker Process, Miki Sakai. It was
really crude, though." Points to Black for discerning the spelling of
Sekai vs. Sakai...