Thanks all. I just haven't been in the field long enough to be familiar with hookless rims. I jettisoned it shortly after the initial post, and replaced it with a 622 wheel I had around. Whatever got cleaned out was probably as mentioned--and old rim strip or melted down sidewalls. It came off strikingly easily with a rotating wire brush. What is history of hookless rims? Makes sense that wire bead would be the answer.