At morpheme boundaries, it seems that [n] assimilated as [m] to a following labial (such as [b]) very late in both Sindarin and Noldorin, much as it did in Primitive Elvish, as noted by David Salo (GS/§4.121). This is most obvious in compound names 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.

It shows up in some older compounds as well, where the n+b combination were the result of middle consonants being lost in large clusters:

• 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 but these are the minority, perhaps intermediate forms or dialectical variations.

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

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 @@@