The final consonants allowed in Quenya words, n, t, l, r, s, are all dentals (Let/425; PE19/104; PE22/62; VT42/7). This means that where labial and velar consonants became final, they must have become dentals. This is true both of original primitive consonants, of consonants that became final after short final [e], [a], [o] vanished, and of single consonants that remained final after final consonant clusters reduced.

Of these, only nasals and voiceless stops were relevant, since other final consonants simplified to these or were modified in different ways. Tolkien described the development of final labial and velar nasals and stops in the Outline of Phonology (OP2) from the 1950s:

This list of “permissible finals”: n, r, l, s, t and nt remained constant in Quenya speech-feeling. Weak vowels were most readily lost when their disappearance left simple n, r, l, s, t finally. And when vowels were lost after other consonants or after groups, as happened in later AQ and PQ under certain conditions (e.g. especially in long compound words), these consonants or groups were changed or reduced to one of these permitted dentals: m > n, and stops > t; d > r; th, þ > t.

Consonantal groups were simplified. Mostly by dropping all but the first constituent: as nd > n; and modifying that if necessary: as mb, ñg > m, ñ > n (PE19/104).

Thus final (voiceless) stops became t, and final nasals became n, though in the case of ñ this only occurred from the reduction of final ñg, since otherwise isolated ñ had already vanished. It is also not clear how common -p > -t was, since in Primitive Elvish Tolkien said that “no certain case of final p, ph, b is cited (OP2: PE19/102-3)”. Nevertheless a final p might arise in Parmaquesta from vowel loss at the end of long compounds.

Conceptual Development: In the Qenya Phonology of the 1910s, Tolkien said final voiceless stops all became dentals (PE21/21-23), though in this period he listed k among the possible final consonants in Early Qenya, so it may have been an occasional exception (PE12/26). See ᴱQ. lak “snare” (QL/51), ᴱQ. piek “pin” (QL/73), ᴱQ. silik “flint” (QL/83), though the stem forms of these words indicate the original endings were either -q or -kk.

In the Qenya Phonology of the 1920s, Tolkien simplified the system of final consonants to just dental n, t, l, r, s, and said that all other consonants became dentals:

All consonants were dentalised. k, p > t; g, b > d; m, n > n; whether alone or in groups; the groups were then all reduced to their most sonorous element, except nt which remained (PE14/69).

He maintained this system going forward. He did not discuss these phonetic developments in detail in the Outline of Phonetic Development (OP1), but there are examples from The Etymologies of the 1930s showing -k > -t and -m > -n:

• ᴹ√PHILIK > ᴹQ. filit “small bird, sparrow” (Ety/PHILIK).
• ᴹ√KEM > ᴹQ. kén “soil, earth” (Ety/KEM).