Poster Stamps became popular in the US around 1912, and appeared earlier in Europe. Most were issued for advertising products, and promoting events. Attached is the first US poster stamp journal, the Poster Stamp Bulletin's first five issues, July through November, 1915. At the time the only exclusive poster stamp publication in America. This club of collectors was promoted in Boy's Companion Magazine, which reported in January 1916 that the Poster Stamp Bulletin had been discontinued. Pioneer Christmas Seal dealer, A. W. Dunning was the vice president of this club.
The October 1925 German poster stamp journal, Die Gelegenheitsmarke, published Dick Green's US Christmas Seal listings (pages 29-30). It is clear that Green is just getting started in his knowledge of Christmas Seals, as he lists a 1908 type 2 coil in error; this seal was from a booklet pane. The high price of 1907 was before a quantity was discovered in 1933. In this same German journal, don't miss the article in English on pages 5-6.
From May 1941 through October 1941, the Christmas Seal & Charity Stamp Society shared a news letter with the Poster Stamp Collectors of America. For more about Poster Stamps see, News 10-30-2017
Check out The Poster Stamp Collectors Club, http://www.posterstampcc.org/1915 Poster Stamp Bulletin.pdf German Posterstamp Journal 10-25.pdf Boy's Companion Magazine, Poster Stamp Bulletin.pdf