Not sure what this is per se or why exactly Saginaw Bay in Michigan is a “DANGER AREA”, but there is text that reads you should consult Civil Aeronautics Admin Notices for changes in aeronautical data on this chart after July 5th 1945 (later revised to 1946). That dates this very large/cool map of Green Bay, WI/Michigan around the WW2 era. It would go great in your study (or living room)

In really good condition. Just some yellowing (probably to be expected)