The word ᴹQ. ambor “breast” appeared in the Declension of Nouns from the early 1930s, derived from ᴹ✶amƀus (PE21/33). This word shows the Early Qenya sound change whereby [[eq|final [s] became [r]]]; in Tolkien’s later writings this change applied mainly to intervocalic [s]. This word also had the unusual development of u to o in final syllables, a sound change Tolkien used for Quenya in the Declension of Nouns but nowhere else.
Conceptual Development: In the Qenya Lexicon of the 1910s Tolkien gave ᴱQ. ambar “breast” with stems ambar- or ambas- (QL/30); the word also appeared in the Poetic and Mythological Words of Eldarissa, but only with the stem ambas- (PME/30). ᴱQ. ambos was glossed “breast” in Early Qenya Word-lists of the 1920s (PE16/136), and in Early Noldorin Word-lists of the 1920s, ᴱQ. ambos (ambost-) appeared as a word related to both ᴱN. bost “back, from shoulder to shoulder” and ᴱN. amoth “shoulder” (PE13/137, 139, 159), the latter with primitive forms ᴱ✶a-mbod-t’ (PE13/137) or ᴱ✶a-mbos-t (PE13/159).
ᴱQ. ambar reappeared in the phrase ᴱQ. níve qímari ringa ambar “the pale phantoms in her cold bosom” from the Oilima Markirya poem written around 1930. Early 1930s ᴹQ. ambor seems to be the last published iteration of this word, as discussed above.
Neo-Quenya: For purposes of Neo-Quenya, I would adapt this word as ᴺQ. ambos (ambor-) “breast, chest” to fit better with later Quenya phonology. It might be an ancient combination of √AM “up” and ᴹ√OS “around”, perhaps with the original sense “upper enclosure (of the body)”.