It seems that [n] assimilated as [m] to a following labial (such as [b]) in both Sindarin and Noldorin, as noted by David Salo (GS/§4.121). This change may have been limited to later compounds, since elsewhere Tolkien said medial nm became nw. This is most obvious in compounds such as:

• S. and N. Calembel “Greenham” (LotR/790; RC/537; WR/437) = calen + pêl.
• S. and N. Celebrimbor “Silver-fist” (LotR/242; PE17/42, 179; Ety/KWAR) = celebrin + paur.
• S. and N. lembas “waybread” < lenn-mbass (PE17/60; PM/404; EtyAC/LEN).

All of these examples of mb must be from later assimilations, because otherwise the mb would have become mm as it usually did medially. Compare this with the following examples, which must have been from early assimilations because the ultimate result is an isolated m:

• S. damen “return, (lit.) go back” = dan + men- (PE17/166).
• ON. Boronmíro > Borommíro > Boromir (Ety/BOR, EtyAC/BOR).

There are a couple of examples that show nb unassimilated:

• S. pen-bed “not pronouncable” (PE17/145) = pen + ped-.
• [deleted] S. danbeth “*answer, (lit.) back-word” (PM/395) = dan + peth.

In notes on the Common Eldarin Article (CEA) from 1969, Tolkien had in·pherth without assimilation as the definite plural form of parth “field” (PE23/139), as opposed to more archaic singular definite em-barth < en(a) parth with assimilation (PE/141). Very likely this in· before labial ph was originally im· with a later modification by analogy with other definite forms. If so, this hints that either (a) the assimilation of n to a following label was no longer an active sound change in Sindarin or (b) was only sporadically applied.

Very likely there was a similar assimilation of [n] to [ŋ] before following velars (such as [g]) as suggested by David Salo (GS/§4.121), but this is difficult to determine orthographically because Tolkien represented both [ŋg] and [ŋ] as ng.

Conceptual Developments: There are no attested examples of nb in the Gnomish of the 1910s. This fact together with examples like G. Tirimbrithla “Tower of Pearl” = G. tirin + brithla (GL/71) indicate that this sound change was part of Tolkien’s conceptions of the language from the very beginning. Lack of clear examples makes determining the details at the earliest stages very difficult, especially since medial mb did not become mm in Gnomish.

GS/§4.121 @@@