WINDSOR — One donated piece of lumber at a time, volunteers this summer put together the Windsor Historical Society’s new post-and-beam sawmill building — the jigsaw puzzle of Bob Brann’s dreams. Now they’ve got another puzzle to put together: the late 19th-century working sawmill.  Donated by an Albion man and disassembled for the move to Windsor, the sawmill will occupy the 96-by-24-foot building, which is the latest addition to the historical society’s prominent spot on the Windsor Fairgrounds. Brann and others involved helped take the old sawmill apart, but putting it back together, he concedes, “could be quite a jigsaw puzzle.”