1956-6px, 1956 U.S. Christmas TB Seals, Imperforate Proof Sheet of 100 seals, pm "W", F, No Gum. Firm paper, fresh, never folded, shipped flat. Scan is an example. Full scan of front and back available upon request.
Issued 1956 by the National Tuberculosis Association (NTA). Designed by Heidi Brandt. Images of boy and girl carrying double-barred cross. Printed by the Western Lithography Co.
Imperforate sheets have no perforations or rouletting at all.
Christmas Seals are also known as TB Charity Seals, and are usually classified as Charity Seals, or Cinderella Stamps.