In Sindarin and Noldorin, where combinations of nasals and voiceless spirants (mph, nth, nch) became final after short final vowels vanished, the spirant became a stop again (mp, nt, nc). The same is true of the combination lth (becoming lt) but not rth. Tolkien described this change in several places:

Similarly in oldest Sindarin, mp, nt, nc were preserved. In North Sindarin they universally became mf, nth, ŋch at the same time as lt, lp, lc and rt &c. > lth, lf, lch. The normal development in North Sindarin was then restopping when final mp, nc, lt but nth, lf, lch, rth, rf, rch remained spirantal and medial nch > ¯ch. Later again nth > nt finally ... In Oss[riandic], West & South Sindarin changes proceeded as far as mph, nth, ŋch. Then lt, mp, np, nc restopped finally (phonetic notes from 1962, PE17/131).

In the Northern dialect (which perished in the course of the war against Morgoth) nt, nc, mp remained unchanged. In the Southern dialects nt, ñk, mp remained when standing finally - or more probably the spirant was re-stopped in this position, for similarly final lth > lt, though rth remained finally (The Rivers and Beacon-hills of Gondor, late 1960s, VT42/27).

S. change only affecting the medial stops and not those that had become final after the loss of all original CE final vowels (-mp, -nt, -ñk remained) ... The stopped forms finally were probably due to re-stopping while the spirants still remained bilabial f, dental þ — they later and by the time of the coming of the Exiles had become labiodental f and interdental þ. Similarly final lþ > lt, though rþ remained medially and finally (Common Eldarin Article, 1969, PE23/137).

Final mp, nt, nc, lt are common in both Sindarin and Noldorin, but finding explicit examples of the restopping is difficult, since Tolkien rarely showed the intermediate changes. There is, however, one example from a deleted entry in The Etymologies:

• ON. maltha > N. malt “gold (as metal)” (EtyAC/SMAL).

Conceptual Developments: Final mp, nt, nc, lt are also widespread in the Gnomish of the 1910s and Early Noldorin of the 1920s. Meanwhile, mf, nth, nch, lth occur medially but not finally, so it is likely this sound change dates back to Tolkien’s earliest ideas of the language, as noted by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.2).

GS/§4.117 @@@

HSG/§2.2, 2.4