it sure seems Japanese but: that 26.4 seatpost is odd. Either it's a heavy gauge tube (and doesn't look like it being all chrome and with nice lugs and forged DOs) or that's a metric seat tube.
Are you sure it's the correct size post?
What's the BB threading?
Suntour GS dropouts were more common on JP frames, but there were a number of EU builders who used them (and Shimano DOs as well)