The root ᴹ√KU(Ʒ) “bow” appeared in The Etymologies of the 1930s (Ety/KUƷ), most likely a later version of ᴱ√KUVU “bend, bow” from the Qenya Lexicon of the 1910s (QL/49). One notable derivative in both periods was G. cû “bow, crescent” and N. cû “arch, crescent”, which regularly appeared as S. cû “bow” in later writings, for example in S. Laer Cú Beleg “Song of the Great Bow” (GL/27; Ety/KUƷ; S/209).
The probably-related root ᴹ√KUB “bow” appeared in the Quenya Verbal System of the 1940s (PE22/102); ᴹQ. nukumna “humbled (?under-bowed)” from this period may also be related (SD/246). In notes from the late 1950s or early 1960s Tolkien gave ✶kūma, Q. cúma and S. cû(f) next to Q. lúva “bow, bight (not for shooting)” < √LUB “bend”, so presumably cúma/cû was “bow (for shooting)” (PE17/122). Finally Q. cúna “bent, curved” appeared in notes associated with the version of the Q. Markirya poem from the late 1960s, along with a verb form cúna- “to bend” (MC/222-223).
These variations make it difficult to determine what Tolkien intended the root to mean, but for purposes of Neo-Eldarin I would assume a base root of √KU(Ʒ) or √KU(H) with perhaps a verbal variant √KUB based on its use in the 1940s, and with the primitive sense “bow, bend”.
An unglossed root in The Etymologies of the 1930s serving as the basis for ᴹQ. kumbe/N. cum “mound, heap” (Ety/KUB). It was undoubtably a later iteration of ᴱ√KUMU “heap up” from the Qenya Lexicon of the 1910s with similar derivatives in both Qenya and Gnomish (QL/49; GL/27). This earlier root also had derivatives having to do with “burden” such as ᴱQ. kumba “burdened, laden”, but based on Gnomish words like G. gûm “burden” and G. gumriol “burdensome” (GL/43), this was probably due to blending with an otherwise unattested root ✱ᴱ√GUMU.
These roots were the basis of G. Cûm a Gumlaith “Mound of the First Sorrow” which later became N. Cûm-na-Dengin “Mound of Slain” (SM/312, LR/147). Eventually this name became Haudh-en-Ndengin “Mound of the Slain”, indicating that ᴹ√KUB was abandoned for this purpose. Indeed, in later writings Tolkien instead gave √KUB as “hide, secrete” instead (PE22/155).