perfect strike in blue on reverse of locally-addressed 1861 cover, franked with 1857 1c Blue tied by blue "Chicago Ills Aug 27" datestamp, additional blue Chicago backstamp as often seen on incoming mail, extremely fine, only two examples of this handstamp are known on drop letters mailed three days apart (August 27 and 30, 1861), a remarkably attractive cover made even more noteworthy as August 27 was the last day of validity for the 1857 issue in Chicago, illustrated in Piszkiewicz (page 152)