Light Action SIS was produced from 1986-1988, so based on the serial number, this should be a 1987 or 1988 model. My database does contain another Sport DLX with an F7 serial number and Light Action SIS. If you can post a pic, I should be able to verify the year based on the paint and decal schemes.

I could see this bicycle hanging around in a shop for a few years, but a decade seems a bit much. The market for entry level road bicycles was drying up and being replaced by ATBs. If it is a very large frame, that would have made it even harder to sell.