In Arthur Koestler's Darkness at Noon, who is "No. 1" supposed to be?

Most people don't know this about Arthur Koestler, the author of the book, but he was at one time, himself a communist.  He was in the German Communist Party, and also lived in the Soviet Union for a time in the late 1920s and early 1930s, witnessing firsthand Josef Stalin's dictatorship, the famine and millions of deaths he caused in the Ukraine, and the fate of communist party officials there who did not obey Stalin no matter what.

By the late 1930s, he was disillusioned with communism and wrote his famous novel, Darkness at Noon, criticizing communism and dictatorships in general.  His "No. 1" character in the book is an obvious reference to the leader of the Soviet Union at the time, Josef Stalin, and the books protagonist, Rubashov, is a collection of characters representing communists he had known in the Soviet Union who Stalin had arrested or executed.