I originally looked at the diamond lug cutouts and fork and thought the same thing "Maybe its a Univega with a fork graft", but then I looked up the serial number and it doesn't have a Miyata serial number scheme. The only scheme at matches is the Miki built Centurion/Sekai/Sakai type. And I looked on the bottom bracket of a centerpull Centurion Pro Tour that I have, and it has exactly the same serial number scheme, and those weren't Miyata made bikes either.

So I'm pretty sure its from the Miki factory, especially since the fork crown and dropouts seem to match up with the 1985 Sekai 2600 (but my mystery bike doesn't have the third bottle cage mounting under the downtube).
I'm mostly curious about the particular tubing used - I'll probably do the obvious thing and shine a flashlight down the seat tube to see if I can spot a seam on the inside.

Steve