Thanks for the heads-up. That's a good-looking frame, even if it's a fixie.
Alas, track frames are not my area of expertise, and I've seen only very few close-up. I'll second @
juvela in that the use of Japanese steel often points to Belgium as the country of origin. The fact that it apparently has a serial number might help narrow down the possible builders, although it could have been added by someone else, as quite a few Belgian shops produced 'blank' frames for others to brand (and sometimes number).
OTOH, given the unusual composition: could it perhaps have come from somewhere behind the former iron curtain?