Shanghai 7621 model watches should have been made since 1976 according to the way Shanghai Watch Factory designated their watch models. This is a quick set day/date watch that is quite sophisticated for a Chinese watch at the time. This is a less common Shanghai watch than those later day/date Diamond watches with a chrome-plated or titanium plated cases. And it appears to be better made than Shuang Ling quick set day/date watches.
It has a well polished movement with a kind of stripe that I only found in those export Chunlei watches. It also has a magnetic shield.