A Sindarin word for “eighth” appearing as {tolthui >>} tollui in notes on numbers associated with The Rivers and Beacon-hills of Gondor from the late 1960s, an adjective form of tolodh “eight” using the suffix -ui (VT42/25; VT42/31-32 note #37). In author’s note 5 of that document, Tolkien said that tolthui was from the Southern dialect of Sindarin, and that in modern pronunciation this had become long voiceless lh (VT42/27). Earlier drafts of The Rivers and Beacon-hills of Gondor had tolhui “eighth” (VT42/10), likely reflecting this modern pronunciation.

It is possible that the revision {tolthui >>} tollui also reflects this change of pronunciation of lth, but Patrick Wynne suggested (and I agree) that it actually reflects the change of Q. {toltea >>} toldëa “eighth” from the same document (VT42/31-32 note #37). This indicates the underlying root was revised from {√TOLOT >>} TOLOD, a root change appearing in other documents from this period (VT47/31-33). If so, then the ancient ld would in Sindarin develop into lð and then into voiced ll, so that tollui with voiced long ll would be the expected pronunciation.

Conceptual Development: The King’s Letter from around 1950 had mutated dolothen “eighth” (SD/129), likely based on earlier [N.] toloth “eight” from The Etymologies of the 1930s (Ety/TOL¹-OTH/OT) with a different adjective suffix -en. The Gnomish Lexicon of the 1910s had G. ungra “eighth” based on {ung >>} G. uvin “eight” (GL/75).