In both Sindarin and Noldorin, a final [l] or [r] after a consonant usually became syllabic, and developed into [-ol] and [-or] respectively, as noted by David Salo (GS/§4.192). There are a large number of examples in both Sindarin and Noldorin:

• ✶oklā [> *ogl] > S. ogol “bad, evil, wrong” (PE17/170).
• ᴹ✶tankla > tachl > N. tachol “brooch, clasp, pin” (Ety/TAK), an etymology that reappeared around 1950, though still marked Noldorin (PE18/100).
• ᴹ✶tekla > tegl > N. tegol “pen” (Ety/TEK).
• ✶satrā [> *sadr] > S. sador “steadfast, trusty, loyal” (PE17/183).
• ᴹ√G-LAM > glambr > glamr > N. glamor “echo” (Ety/GLAM; EtyAC/GLAM).
• ᴹ✶tatharē > tathrē > N. tathor “willow-tree” (Ety/TATHAR).
• ON. ndakro > dagr > N. dagor “battle” (Ety/NDAK), a word that reappeared in Sindarin.

There are, however, a couple examples where this sound change did not occur:

• ✶mikrā > S. megr “sharp-pointed” (WJ/337), marked with a “*” by Tolkien so perhaps an archaic form.
• ᴹ✶lakra > N. lhagr “swift, rapid” (Ety/LAK²).

These may be archaic forms. There are also a couple more examples where it seems the vowel that developed was not [o]:

• ✶nathlo > S. nathal “guest” (PE17/141).
• ᴹ✶idrē > N. idher “thoughtfulness” (Ety/ID).

Here it is more difficult to guess Tolkien’s thinking. More likely this represents some kind of earlier metathesis of liquids and vowels, such as: idrē > *ider > idher.

Conceptual Developments: The analysis of these changes in the early stages of the language are complicated by the fact that syllabic [ḷ] and [ṛ] were ordinary vowels in the Primitive Elvish of the 1910s and 1920s. There are also quite a few examples where a final [l] or [r] did not develop a vowel after another consonant in the Early Period:

• G. cingrafindl “pigtail” (GL/26).
• G. fingl “tress” (GL/35).
• G. aithl “spring” < ᴱ✶ektḷ (GL/18), a word that reappeared in the Early Noldorin word lists (PE13/136, 158).
• ᴱN. naugl “dwarf” (PE13/150; PE15/62).
• G. gadr “joint, link of chain” (GL/36).
• G. aithr “thorn” (PE13/108), a word that reappeared in the Early Noldorin word lists (PE13/136, 155).
• ᴱN. caifr “flea” < ᴱ✶kamp(r)u (PE13/140; PE14/66).
• ᴱN. taichr “prop, support” < ᴱ✶tank’rú (PE13/153).

Thus, it seems that in most cases a syllabic final [-ḷ] or [-ṛ] simply remained in the Gnomish and Early Noldorin versions of the language from the 1910s and 1920s.

GS/§4.192 @@@