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Spring boarding from what @Eiríkr Útlendi said, there's no evidence that Japanese has ever had /ji/ <yi> or /wu/ <wu> as sounds separate from /i/ and /u/ as far back as linguists can reconstruct.
Only through wiktionary, en.wiktionary.org/wiki/%E3%81%A3%E3%81%BD%E3%81%84#Etymology which I now notice doesn't have a citation. Perhaps that was why it was easy to find. But if I see anything else backing it up I will comment again with the update.
@boiko It's easy enough to find to purchase but outside my current budget. So I'll just put it on my growing reference book wishlist. Thank you for the suggestions
