1921-1, WX28, 1921 U.S. Christmas TB Seals, block of 4. Fresh, full gum, F, MNH. Issued 1921 by the National Tuberculosis Association. Designed by George V. Curtis. Image of Santa and chimney.
This is Type 1 of the seal, green, blue and red, 19x19mm, with perf 121/2. Scans of front and back provided upon request.
Christmas Seals are also known as TB Seals, and are usually classified as Charity Seals, or Cinderella Stamps.