In Noldorin, there are several examples where a final [-vn] became [-von]:

• ᴹ√KYÉLEP > celefn > N. celevon “*of silver” (Ety/KYELEP).
• ᴹ✶lebnā > N. Lhevon “Elves remaining behind” (Ety/LEB).
• ᴹ√DARÁM > dravn > N. dravon “hewn log” (EtyAC/DARÁM).

The last example was deleted and replaced by N. drafn (Ety/DARÁM), and there are a fair number of other examples where final [-vn] was preserved:

• N. cevn “of earth, earthen” (Ety/KEM).
• N. dofn “gloomy” (Ety/DUB).
• N. rhafn “wing (horn)” (Ety/RAM).
• N. thafn “post, wooden pillar” (Ety/STAB).
• N. tofn “lowlying, deep, low” (Ety/TUB).

It’s not clear whether this sound change could apply to Sindarin; we only have one example of final [-vn], and it was preserved: S. tavn “a thing made by handicraft” (PE17/107). But certainly [-von] is easier for an English speaker to pronounce. Since we only have a single example, it could have been a sporadic change in Sindarin as it was in Noldorin.

It may seem that [-vn] > [-von] is a very specialized change, but aside from [-rn] (which was easily pronounced and remained unchanged), [vn] is the only other surviving combination of a consonant with [n] in Sindarin/Noldorin:

• [pn], [bn] > [vn] (VT42/26); similarly [mn] > [vn].
• [tn], [dn] > [nn] (VT42/26).
• [kn], [gn] > [gn] and later vocalized; > [in] in Sindarin (VT42/26), or > [in], [un] in Noldorin (PE22/39-40).
• [ɣn], [ŋn] > [gn] (PE22/26) and thus fell in with the above.
• [tʰn] > [ttʰ] > [θ], and probably similar changes for other aspirates
• [ln] > [ll] (PE19/100).
• [sn] > [nn] (PE22/35).

Thus, this change was analogous to the syllabification of final [r], [l] after consonants, producing the same vowel [o]. However, final n-syllabification seems less universal that the l/r-syllabification, happening only sometimes, and sometimes with [-vn] preserved.

Conceptual Developments: There is Gnomish example of [-vn] > [-von], as noted by Roman Rausch in his Historical Phonology of Goldogrin (HGP/§3):

• ᴱ✶uƀna > G. uvon “eight” (GL/75), but with later form uvin.

As suggested by Roman Rausch, this seems to be part of a larger trend in Gnomish of final syllabic [-ṇ] becoming [-on], for example:

• G. pilon “arrow, dart”, plural pilnin (GL/75) vs. ᴱQ. pilna.

Thus, it seems that n-syllabification was similar in Gnomish as it was later in Sindarin/Noldorin and produced the same vowel [o], but there were a few more possible combinations (such as [-ln]). As with Sindarin/Noldorin, final -rn is very common and remained unchanged, but there is at least one other example where syllabification did not occur: G. tathn “number” (GL/69).

In the Early Noldorin of the 1920s, there are no examples of -fon or -von, but a couple examples of -fn [-vn]: ᴱN. lafn “animal” < ᴱ✶labna (PE14/70) and ᴱN. nofn “down, set” (PE13/151). Perhaps final n-syllabification did not occur in the 1920s, but this could simply be lack of examples, as with Sindarin.