The serial number indicates that the frame was manufactured by Yamaguchi of Japan, during August 1980. That's in the gray region of the calendar year, where it could be either a 1980 or 1981 model. Given the Yamaguchi origin, era and the OP's location, my prime candidate would be a Norco. During this era, their 600EX model was the Magnum SE. Here's a photo showing one in NOS condition. Like the subject bicycle, it has no braze-ons save for the chain stay cable stop. It also has chromed dropout flats, a sloping crown fork, same lugs, etc. It appears to be a very good match, though the owner mistakenly identifies as a 1985 model manufactured by Nishiki. The only anomaly is the presence of the decal for Tange #2, as that tubeset would normally use a 26.6-26.8mm post. The OP's size is more indicative of Ishiwata 022, though that could a year to year variation.
