The Stamp Centenary Exhibition was an international stamp exhibition. It was held at Lancaster House in London from 6–11 May 1940, under the auspices of the Royal Philatelic Society. It was in aid of the Lord Mayor's Red Cross and St. John Fund. Penny Black reprints (with full gum) were reprinted by Waterlow and Sons for the exhibition.
References
Stamp Centenary Exhibition 1940 Wikipedia(Text) CC BY-SA