Certified plate proofs are the last printed proof of the plate before printing the stamps at the Bureau of Engraving and Printing. These plate proofs are each unique, with the approval signatures and date. For postal scholars these plates provide important production information in the plate margin inscriptions, including guidelines, plate numbers, and initials of the siderographer, or person who created the plate from a transfer roll.
Plate No. 21178
Denomination: 50c
Subject: "Graf Zeppelin", Federal Building at Chicago Exposition and Hangar at Friedrichschafen
