The best place to start is the serial number. That may allow us to identify the manufacturer and year. We know that the Japanese Schwinns were made by Panasonic and their serial number format is easily recognizeable. However, that brake may not be original, as the eras for all the components do not match. SunTour GT dropouts without bottle bosses, lever bosses or top tube cable tunnels would generally place it in the 1970s and at least mid-range. What size seat post does it take?