[Galin]

In his Foreword to The Book of Lost Tales Christopher Tolkien assumes the same thing that Robert Foster had published in his Complete Guide to Middle-earth, that Quenta Silmarillion was no doubt one of Bilbo's translations.

"So also I have assumed: the 'books of lore' that Bilbo gave to Frodo provided in the end the solution: they were "The Silmarillion". But apart from the evidence cited here, there is, so far as I know, no other statement on this matter anywhere in my father's writings; and (wrongly, as I think now) I was reluctant to step into the breach and make definite what I only surmised."

Christopher Tolkien, Foreword, The Book Of Lost Tales volume I

Richard Plotz talked with Tolkien on November 1, 1966, just after the second, revised edition (Allen and Unwin) had been published in October, after the Ballantine Books revised edition had been published as well.

Tolkien tells him that one of the snags delaying publication of The Silmarillion is its quasi-biblical stle, which Tolkien considers "his best, but his publishers disagree. Another problem is that of finding a story line to connect all the parts. At the moment, Professor Tolkien is considering making use of Bilbo again ... perhaps the Silmarillion will appear as his research in Rivendell."

Hammond And Scull, Chronology

I find his phrasing interesting, as it looks like Tolkien had already added his Note On The Shire Records (revised edition), which included that Bilbo's Translations From The Elvish were "almost entirely concerned with the Elder Days". And JRRT had added in Appendix A (revised edition) that the ancient legends of the First Age were Bilbo's chief interest.

Admittedly Quenta Silmarillion is not specifically noted in either of these descriptions from the second edition, but in any case this is how Plotz put it.