It is rather clear from the prologue to The Fellowship of the Ring, specifically the "Note on the Shire Records," that the Lord of the Rings account of the third age comes from information primarily in "the Red Book of Westmarch." This book originated as "Bilbo's private diary" and expanded by Frodo "during S.R. 1420-1." There were also "annexed to it and preserved with it ... three large volumes ... that Bilbo gave to him [Frodo] as a parting gift." Then additionally, another book, "a fifth containing commentaries, genealogies, and various other matter concerning the hobbit members of the Fellowship."
So to summarize thus far, the five volumes contained:
- Vol 1. Bilbo's diary (and Frodo's added story)
- Vols 2-4. Histories gained from Bilbo
- Vol 5. Histories related to the hobbits
The prologue also notes that the most important copy of the Red Book was "completed in S.R. 1592 (F.A. 172)," one which was "an exact copy in all details of the Thain's Book in Minas Tirith ... made at the request of King Elessar," a copy of the book delivered to there by "Thain Peregrin when he retired to Gondor in IV 64." The later (IV 172) copy "received much annotation, and many corrections, especially of names, words, and quotations."
So it seems the latest compiling is IV 172, though possibly some annotations later.
Now that is all said to ask about this statement from The Two Towers, Book IV Ch. 4, "Of Herbs and Stewed Rabbit," where it is noted (emphasis added):
The Mûmak of Harad was indeed a beast of vast bulk, and the like of him does not walk now in Middle-earth; his kin that live still in latter days are but memories of his girth and majesty.
So who wrote this and when?
- If this is Frodo's hand, since it is part of his story, then it was written S.R. 1420-1. But would he be using such language as "now in Middle-earth" and "latter days" (and would he even know that no Mûmak still lived in Middle-earth)?
- If this is a later annotation by a copiest, when was it added?
At what time were the Mûmak effectively extinct from Middle-earth (in the size of their grandeur) to such a point that one actually knew none were left (i.e., they would have been extinct for some time I suspect), and hence what period of time might this note have been added to the history at this point in the story?
As I note in a comment on the first answer to be given, I do not think it is valid to consider this information merely an insertion by Tolkien, for the quote states "the like of him does not walk now in Middle-earth" (emphasis added). That phrasing of a "present time" (from the author's perspective) would not be how an insertion phrased by Tolkien would be stated, since his "now" is long past any "now" of Middle-earth (that is, Tolkien exists in Earth, not in Middle-earth). So the reference to what "now" (at that time) was existing in Middle-earth makes no sense as added by Tolkien (the much later translator) nor is it phrased in a way that Tolkien (as omniscient interpreter) would be expected to write it since it has the voice of one living in Middle-earth (rather, if so, rewording something like "and the like of him did not walk in the latter days of Middle-earth, his kin that live still are but memories of his girth and majesty").
So the best explanation is that Tolkien is "translating" an older text written by a Middle-earth dweller that had knowledge of a later period of Middle-earth in which that dweller was then existing in a "now" moment being referenced.
Which is why I asked the question.