Alright, I'm back! And sorry for the late reply. In Hammond and Scull's Reader's Guide (Pg. 100, under the History entry for 'Of Beren and Luthien'):
>The chapter 'Of Beren and Luthien' in The Silmarillion was based for the most part on the texts of the Quenta Silmarillion of the 1930s, mainly on a rejected first fair copy as far as the point where Felagund gives the crown of Nargothrond to Orodreth, but with some elements from the complete fair copy which was the source for the rest of the chapter. Christopher Tolkien also took from the Grey Annals a short passage describing Barahir's refuge, and several short phrases which elucidated points of importance. He took the account of Gorlim's treachery from the revision of c. 1950, and inserted thirty two lines from the Lay of Leithian describing the contest between Felagund and Sauron (covered in only on sentence in the Quenta Silmarillion). See further, discussion in The Lost Road and Other Writings, pp. 295-306; The Lays of Beleriand, p. 196; and The People's of Middle-earth, pp. 318 and 369, and p.372, note 8.
So, there you have it! I've just recently purchased this book by Hammond and Scull, and the accompanying chronology, and they're awesome! They say further that the evolution of the story was virtually complete by the end of 1937, with the completion of the Quenta Silmarillion, in which Tolkien had some difficulty keeping it succinct. He later revised the Lay, in about 1950, and worked on a full prose version related to the revised Lay. He added a few details in the Grey Annals but nothing drastic and the full story was pretty much complete by 1937, as I said.