Symbol LATIN SMALL LETTER E WITH FISHHOOK used for rhotacized central vowel [ə˞] in John Samuel Kenyon, American pronunciation: a text-book of phonetics for students of English, 1924-1932 (1st-5th editions) before it was replaced by Kenyon symbols ɚ and ɝ in 1935 (6th edition).
Own work
Denis Moyogo Jacquerye