Lee,
Might be a seatpost off a MKI Ultra Burner since I've seen them popped out of a fair few of them.
Koizumi were primarily 2nd stage fabricators, much of their tubing came from Tange which they simply cut to shape, stamped and welded up. They did in some cases produce their own framesets, but on rare occasions. One sure thing was their quality products and their welding was generally considered better than most of the competition.
southend_styler, the MKI Pro Burner and some Aero Pro's came from the Koizumi factory and pretty sure they did a Hutch, but to name a few more.
Mark