Gosh, there are too many suspects here in this 1937 pic to do at once. Let's see. The bottom row has someone who used to writer with his brother, someone whose son had almost the same name, and an agent/editor who ended up in the comics.
[Well, you guys named most of them but let's make it clear. Back row, left to right: Jack Williamson, L Sprague de Camp, John D Clark, Frank Belknap Long, Mort Weisinger, Edmond Hamilton, Otis Adelbert Kline. Front row: Otto Binder, Manly Wade Wellman, Julius Schwartz. Not shown: Jommy Cross, John the Balladeer, Adam Link and two Bug-Eyed Monsters.