It's an ETR (see the OP's other thread). There's nothing camera specific here. Is the strap a Bronica strap? It is likely that the ring is steel, not aluminum. I can't tell if it is a split ring or something else, but it is likely that the secret to opening it is hidden by the fabric and plastic strap cinch.
Anyway, I don't think worrying about scratching the camera with the strap is particularly important. Virtually all straps designed to fit into that kind of camera lug with a hole use a metal (typically steel) ring, because that's what is strong enough.