After the loss of vowels at morpheme boundaries in both Sindarin and Noldorin, where a nasal appeared before a voiceless spirant (f, th, ch, s) and formed clusters of three or more consonants, then that nasal vanished. This sound change was noted by David Salo, though he more conservatively assumed the change was limited to clusters ending in a liquid r, l (GS/§4.113). Tolkien himself described this change in a few places:

As in Sindarin general[ly] m, n tended to be lost before another consonant, camprû > camfrū > cāfru (1962 notes, PE17/131).

... since the spirant appeared also in NS [North Sindarin] where the nasal was lost. This occurred in S. when another consonant (l, r) followed the stop: so CE *antr, *ankl for example became aþr, aχl (without lengthening of the vowel), as in cathr “carpenter” (NS cathor) < *kantrō ... tachl “large pin or brooch” (NS tachol) < *tanklă (1969 notes, PE23/137).

These notes indicate such nasal-loss before voiceless spirants was fairly common, but all the attested examples of such nasal-loss in three-consonant clusters involve a liquid, as in Salo’s phonetic rule (GS/§4.113). Very likely the most common three-consonant clusters ended in an l or r. There are a number of examples of this phonetic development in both Sindarin and Noldorin:

• ✶ninkwiraite > ni(m)phraed > S. niphred “pallor” (PE17/55), also N. nifred of similar origins (Ety/NIK-W).
• ✶tanklā > tankla > tachl > N. tachol “clasp, brooch” in both the 1930s and around 1950 (Ety/TAK, PE18/100).
• ✶kwentrō > pentro > N. pethro-n “reciter” in both the 1930s and around 1950 (Ety/KWET; PE18/50, 100).
• ᴹ✶la(n)sro-ndo > N. lhathron “listener” (Ety/LAS²), with later cluster sr > thr.

There are also examples that show nasal-loss before only a single voiceless spirant such as Arathorn < aran-thorn (PE17/113) and [N.] ifant = în + pant (Ety/YEN). See how nasals vanished before morpheme boundaries for further discussion.

Conceptual Developments: Lack of examples makes it hard to determine whether this sound change occurred in the Gnomish of the 1910s, but there are at least a couple examples of something like this phonetic development in the Early Noldorin of the 1920s, though at this conceptual stage it seems the nasal vocalized to [i]:

• ᴱ✶tank’rú > ᴱN. taichr “prop, support” (PE13/153).
• ᴱ✶kampru > ᴱN. caifr “flea” (PE13/140; PE14/66).

Coincidentally the primitive form of the last example is essentially the same as the first Sindarin example cāfru quoted above, written over 30 years later.

GS/§4.113 @@@