What indeed had Tolkien published by this time? As I said, not The Hobbit, nor had he yet published (nor even delivered) his famous lecture, “Beowulf: The Monsters and the Critics”. By 1935, Tolkien would have been chiefly known for his and E.V. Gordon’s edition of Sir Gawain and the Green Knight (1925); his Middle English Vocabulary (1922); two substantial essays “Sigelwara Land” and “Chaucer as a Philologist” (both 1934), and several smaller ones; his reviews of new publications in philology for The Year’s Work in English Studies (1924–7); and a handful of published poems.
But here, Lewis refers to an alliterative poem, and it seems to me he has a specific work-in-progress clearly in mind. If so, which one was it? There are several possibilities, among them the following.
Still during Tolkien’s lifetime, J.B. Bessinger and S.J. Kahrl decided that Lewis must have been thinking of The Homecoming of Beorhtnoth Beorhthelm’s Son, which Tolkien published in Essays and Studies in 1953 . Maybe, but Beorhtnoth is really a drama, though one written in verse form. In any case, Lewis was clearly very wrong to think it would “soon” appear, “soon” being an adverb seldom, if ever, applicable to Tolkien. By 1950 — if not long before — Lewis had learned better, calling Tolkien “that great but dilatory and unmethodical man” .
But I’m inclined to think Lewis might have been referring to something else. The emphasis in Lewis’s remark is squarely on a forthcoming alliterative poem by Tolkien. Lewis might have had in mind Tolkien’s Lay of the Children of Húrin. Tolkien worked on this long alliterative poem in the 1920’s, but he never managed to finish it. Tolkien shared with Lewis parts of The Lay of Leithian, another work on which he was engaged during roughly the same years. This was a rhyming, not an alliterative work, but they may have also discussed the great alliterative poem that had occupied his imagination during the same decade. It is unfortunate he never completed either of the great lays, but Lewis said it best when he wrote of Tolkien: “His published works (both imaginative & scholarly) ought to fill a shelf by now: but he’s one of those people who is never satisfied with a MS. There mere suggestion of publication provokes the reply ‘Yes. I’ll just look through it and give it a few finishing touches’ — wh[ich] means that he really begins the whole thing over again” .
There’s another possibility, this time something that Tolkien actually did finish. During the late 1920’s and early 1930’s, Tolkien worked diligently on the two companion poems, Völsungakviða en Nýja [“The New Lay of the Völsungs”] and Guðrunarkviða en Nýja [“The New Lay of Gudrún”], each executed in hundreds of meticulously crafted Eddic fornyrðislag stanzas. The Old Norse alliterative meter was, for all intents, nearly identical with the Old English. Indeed, immediately following his reference to Tolkien’s poem, Lewis writes that “the moment seems propitious for expounding the principles of this meter to a larger public than those Anglo-Saxon and Old Norse specialists who know it already”. Tolkien was a “specialist” in both, and fortunately for posterity, he finished the two Volsung poems — though if this was the work Lewis had in mind, then “soon” turned out to be almost seventy-five years! It was only last year that The Legend of Sigurd and Gudrún was finally published.
This is not, by the way, the only time Lewis promised a forthcoming work by Tolkien. I suppose he may, in part, have been attempting to motivate Tolkien further by putting the promise of the work into print, thereby exerting a friendly pressure on Tolkien to “get on with it”. The most famous example is in the preface to That Hideous Strength, where Lewis wrote in 1943: “Those who would like to learn further about Numinor [sic] and the True West must (alas!) await the publication of much that still exists only in the MSS. of my friend, Professor J.R.R. Tolkien”. If such was Lewis’s aim, it seems Tolkien got the point; he wrote to a correspondent in 1955, “[Lewis] used the word [“Numinor”], taken from my legends of the First and Second Ages, in the belief that they would soon appear. They have not, but I suppose now they may” . Of course, they didn’t — not for another twenty years.
But getting back to alliterative verse, I’d like to close with an amusing bit of Lewis’s own (of which his essay is full of examples). This one, which refers to Tolkien by name, was probably concocted out of an old bar anecdote :
We were talking of dragons, Tolkien and I
In a Berkshire bar. The big workman
Who had sat silent and sucked his pipe
All the evening, from his empty mug
With gleaming eye glanced towards us;
‘I seen ’em myself’, he said fiercely. 
 Lewis, C.S. “A Metrical Suggestion.” Lysistra, Volume 2 (May, 1935): 13–24. Reprinted as “The Alliterative Metre” in Rehabilitations and Other Essays. Oxford: Oxford University Press, 1939: 117–32. Reprinted again in Selected Literary Essays. Ed. Walter Hooper. Cambridge: Cambridge University Press, 1969: 15–26. The curious reference appears in the opening paragraph of the essay.
 Bessinger, Jess B., and Stanley J. Kahrl, eds. Essential Articles for the Study of Old English Poetry. Hamden, CT: Archon, 1968, p. 316 note 1. Bessinger and Kahrl reprint Lewis’s essay on pp. 305–16 of their book. Much more recently, Carl Phelpstead pointed to a note — “Alliterative Metre” (1969), p. 15 note 2 — in which Walter Hooper throws doubt on the assumption that this could be Beorhtnoth and gives Tolkien’s own guess that Lewis probably had in mind “The Fall of Arthur” (incomplete and still unpublished). See Phelpstead, Carl. “Auden and the Inklings: An Alliterative Revival” in JEGP, Vol. 103, No. 4 (October, 2004): 433–57, p. 441.
 Lewis, W.H., ed. Letters of C.S. Lewis. Rev. and enlarged ed. London: Geoffrey Bles, 1966. Reprinted San Diego: Harcourt Brace, 1993, p. 399.
 Ibid., p. 376.
 Carpenter, Humphrey, ed. The Letters of J.R.R. Tolkien. Boston: Houghton Mifflin, 1981, #169.
 The Letters of J.R.R. Tolkien, p. 389.
 Lewis, “Alliterative Metre” (1939), p. 122.