Can someone help identify my bike frame?

I needed to replace my old fixie frame with something lighter, and with a standard BB shell. So I came across this one online. The seller bought it off a friend who stripped and painted it, so any knowledge of its provenance won’t come from there.

What drew me to it was the fork. Clearly quality steel rather than piece of pipe bent into shape like you see on cheaply built single-speed frames.

It has a serial number on the BB shell (Y5K2698), but googling for this only points weakly to it being Japanese, possibly built for the American market, possibly in 1985. But I suspect it’s rather more recent than that.

There are no bosses for shifters, cable stops or eyelets for panniers or fenders, only one set of bidon cage bolts on the downtube, and the seatstays attach to the side of the seattube rather than under the seatpost clamp. The dropouts are semi-horizontal and about 20mm deep. No engravings on the dropouts as far as I can see [edit: dropouts are marked as Tange, so I guess the whole frame is tange steel]. Purpose-built for fixed or single speed I would guess, due to the lack of bosses or derailer hanger, but curious that it has forward facing horizontal dropouts rather than track ends.

Here’s an image of the whole bike, and here’s a gallery of details (sorry- I should have stripped it for proper photos of the frame only, but I just took some snaps on my way out the door to work!).

If anyone can help I’d be grateful for your knowledge. Won’t change anything of course; the bike is still a pleasure to ride irrespective of its provenance.