ᴹ√POR is an unglossed root in The Etymologies of the 1930s with the derivative ᴹQ. pore “flour, meal” < ᴹ✶pori (Ety/POR). It is a later variation on a root ᴱ√POL-I from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. pole “oats” and ᴱQ. polu “kernel” (QL/75). ᴱQ. pole appeared again in the Early Qenya Grammar of the 1920s as “grain”, and again as ᴹQ. pole “meal, flour” in the Declension of Nouns (PE21/12-13). The shift of pole >> pore in The Etymologies of the 1930s seems to be temporary, as primitive ✶poli “meal” reappeared in Common Eldarin: Noun Structure from the early 1950s (PE21/80).
These words for “meal” collided with the root √POL in notes probably from the early 1960s, where Tolkien said:
> √POL. This cannot refer to strength. (Too obvious a reminiscence of [Latin] pollens); also it does not account for poli- “meal”, grist. √POL- should have senses “pound up”, break up small, reduced to powder etc.
This changes to the root √POL seems to be transient, however, since in another note from around 1968 Tolkien said:
> Q. pol, large, big (strong). polda, big. DELETE pole “meal”! Make it mule (PE17/115).
This indicates that the word for “meal” was transferred to the root ✱√MUL, itself probably a restoration of the early root ᴱ√MULU “grind (fine)” (QL/63). See that entry, and the entry for √POL, for a more detailed discussion of those roots. As for “meal, flour” roots, it seems the development was: 1910s ᴱ√POL-I >> mid-1930s ᴹ√POR >> early 1960s ᴹ√POL(I) >> 1968 √MUL.
Neo-Eldarin: For purposes of Neo-Eldarin, I think √POL(I) can no longer be used for “meal”, with √MUL being more suitable for that purpose, but I think 1930s ᴹ√POR might be salvageable for use as “flour”.