In a reply to my blog entry of the 19th of January Jack Windsor Lewis points to Bloch & Trager's Outline of Linguistic Analysis of 1942, where on page 47 they mention the term 'diagonals' (i.e. forward slashes), which they use to enclose phonemes. ɥere is the oriɡinal quotation from their book:
One year earlier, in 1941, Trager & Bloch published an article in Language (vol. 17, p. 229) with the title "The syllabic phonemes of English". Here's the screenshot of the footnote in which they use the term 'diagonals'
Can anyone come up with an antedating?