I agree on the 85 estimate too, I think that's the only year they made the 600. I have the same frame in a much smaller size (46cm?) that I'm going to be moving along at some point. They have very nice details like the "investment cast" lugs and in-chainstay cable routing. Mine still shows some of the original "600" on the chainstay protector as well, just fyi for further evidence as a 600.

Kind of a weird tubing label setup on your frame. I don't have mine in front of me right now but it definitely doesn't have the Tange label and while the catalog page says 531cs, I think the tubing label on mine is actually a 531c. Maybe they ran out of cs labels?

Original catalog page: