(Advertising Covers) Scientific American Patent Agency, gray all-over illustrated ad design on cover bearing 2¢ black, E. grill (87) tied by quartered cancel duplexed with "New-York City Mar 26" cds to local street address, red New-York City (3.24) backstamp; no flap, light edge wear, F.-V.F. Estimate  $200 - 300. $0 (Image)