Often it is possible to to narow down the era within a year or two based on components and decal schemes. While the resolution is a bit low, the pictured Semi-Pro appears to have the original generation SunTour Cyclone derailleur, putting it 1975-1980. The presence of what appears to the round headbadge, puts it into the latter end of that range. If you can post a good resolution copy of the spec page somewhere, or send it to me, I may be able to narrow it down further.