The markings are very generic. Then you have the fact that a lot of name brands that made some of their bikes, had bikes made by others. So is a Schwinn made by Giant a Giant or a Schwinn? Then you have all the private label bikes from that era, sold by chain stores like REI and others, that never made a single bicycle.
At best, your serial number MAY give a glimpse into who made it. It will not tell you model or brand that it was sold under. I have many, many bikes made by others, like Panasonic built Schwinns, Miki built Sakai, goes on and on. So the serial numbers on those bikes all represent the actual maker, and not the brand or model that the bike was sold under.
If its a private label bike, no one is following those brands, and you will not find repop decals for them.