The AR derailleurs should put the Cosaro between 1981-1986. Thumbshifters on a bicycle with 27" wheels is a strange mix. Curiously, I have specs for for a Corsara Ontra road bicycle from 1986 with SunTour AR derailleurs and Tange 900 tubing. It sold for $230 US.

If you want to narrow down the age a bit further, refer to the Components page of the Vintage-Trek website for decyphering the component date codes.

Typically, a bicycle with a 20.5" top tube would be a small frame, with something around a 20" seat tube. This would be far too small for someone who is 6'1". Most people your height would be looking for a frame with about a 24" seat tube. Typically, frames in this size would have a top tube about 22.5".

Are you sure that you are measuring the tube correctly? The length of the tube is measured for the center of the intersecting tubes. In this case, the top tube would be measured along its centerline, from center of the seat tube, to the center of the the head tube.