The root ᴱ√MULU “grind fine” appeared in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. mulda “powdery” and ᴱQ. mulma “fine flour” (QL/63). In the contemporaneous Gnomish Lexicon, G. bloss “wheat” was derived from mol- (GL/25). This root probably as served as the basis for ᴹQ. mulo “dust” from the early 1930s (PE21/10-11). The root √MUL may have been restored later on, as evidenced by the word Q. mŭle “meal [ground grain]” from a list of words having to do with “large & small” from 1968, replacing Q. polë of similar meaning (PE17/115).
Primitive elvish
mōl
noun. slave, slave, [ᴹ✶] thrall
mō
root. labour, be afflicted
mul Reconstructed
root. grind (fine)
mā-limi
noun. wrist, (lit.) hand-link
(m)bol
root. *labour
khaw
root. big
This root first appeared as unglossed ᴹ√MŌ in The Etymologies of the 1930s with derivatives like ᴹQ. mól/N. mûl “slave, thrall” and ᴹQ. móta-/N. muda- “labour, toil” (Ety/MŌ). The root √mō was mentioned again in notes associated with 1950s Quenya prayers, where it was glossed “labour, be afflicted” along with verb Q. moia- of similar meaning and noun (Q. or primitive) mōl “slave” < mō-l; Tolkien went on to suggest √mol might be a better root, though he marked it with a “?” (VT43/31).
In notes on “large vs. small” from 1968 Tolkien had (root or verb?) mol- “labour” and (noun?) mōle, followed by MBOL/BOL with derivative Q. molda “big, large”, but this note was crossed through and √(M)BOL “✱large” did not appear in the next list of the large/small roots (PE17/115). Thus I think √MO(L) reverted back to the meaning “labour”.