Originally Posted by T-Mar

A 25.4mm seat post would confirm a Sekai 500, as that is down into the plain gauge, hi-tensile steel range and the Sekai 400 and 500 were the only two models with that grade of tubeset during this era. However, the 400 was typically equipped with Shimao derailleurs and a steel, cottered crankset, while the 500 was SunTour equipped and got an aluminum, cotterless, Sugino crankset as on your bicycle. I'd expect the outer diameter on the seat tube to be 28.6mm