This era of Fastback was built when the Schwinn name was being bought and sold and they were outsourcing their builds.

I guess it would be like the Waterford era of Schwinn Paramounts or when Yoshi Konna was building Allez framesets for Specialized, you were getting a lot of bang for the buck.

edit: Steve, is there a "Made in U.S.A." sticker anywhere on the frame? I could be entirely wrong about the frame's origins, this is just stuff I've gleaned from the internet over the years