The noun G. alm appeared in the Gnomish Lexicon of the 1910s with the glosses “the broad of back from shoulder to shoulder, the back, shoulders” along with a deleted variant {aldum} (GL/19). In this document it was derived from primitive ᴱ✶alđam-, and was clearly related to ᴱQ. al(da)mo “broad of the back” from the contemporaneous Qenya Lexicon under the early root ᴱ√ALA² “spread‽” (QL/29).

The forms alf, alaf appeared in Gnomish Lexicon Slips glossed “the broad of the back from shoulder to shoulder” and with the primitive form ᴱ✶aldǝmā (PE13/109). ᴱN. alm appeared in Early Noldorin Word-lists from the 1920s with the incomplete gloss “back (from shoulder ...” (PE13/136). This 1920s document elsewhere had deleted forms {alf, alw} with the full gloss “the back from shoulder to shoulder” (PE13/136).

The deleted forms alf, alw are likely to be later than alm reflecting the Early Noldorin sound change of non-initial m to v (spelt f finally), a sound change that was not a feature of Gnomish as it appeared in the Gnomish Lexicon. These two deleted forms were revised to ᴱN. bost “back from shoulder to shoulder” (suffixal form -mmost), which in turn was related to ᴱN. amoth “shoulder” from primitive ᴱ✶a-mbod-t’ (PE13/137, 139); bost seems to be the last of the “back” words in these Early Noldorin Word-lists.

Neo-Sindarin: The early root ᴱ√ALA² “spread‽” was the basis for ᴱQ. alda “tree”, which in later writings was derived from ✶galadā. Since it seems the early root ᴱ√ALA² >> √GAL, Gnomish alm and Early Noldorin alf might adapted as ᴺS. galf “back, shoulders” for purposes of Neo-Sindarin, with an original sense of “spread (of the body)” referring to just the shoulders but later the entire back. The sense of the later root √GAL was “grow (like plants), flourish” rather than “spread”, so this is a bit of a reach semantically, though not impossible since ✶galadā referred to broad, spreading trees as opposed to ✶ornē for tall trees (NM/349). I would use ᴺS. galf for the “back” of a body and the back of the shoulders collectively. I would use ᴺS. amoth for an individual shoulder.

Strictly speaking, ᴱN. bost is a later word for “back” than alm >> alf, but I can’t figure out a way to incorporate it into the etymological framework of later versions of Tolkien’s languages, which is why I recommend ᴺS. galf “back” instead.