Spending the vast majority of my studies in the Koine, and being in favor of the system of eight cases, I was hopeful that perhaps I would find traces of the original ablative form the further back in time I could go searching in Greek sources. When I started working on the first line of Hesiod’s “Works and Days” last week, I think I may have found such a trace in the –θεν morpheme of Πιερίηθεν, from Pieria. I shouldn’t have been surprised when I saw this—it even survives into the NT with ἄνωθεν, from above.
Now obviously a lot more research is needed to establish such a conclusion as fact. But I am finding several resources which discuss the matter--Monro's Homeric Grammar being one--so I'm not the only person who's noticed the issue.