I think you're correct, I don't think the locating rings actually need to be magnetic. However, where the rings fit, I suspect, is reinforced by an inserted metal strip, so that potentially assists in holding any magnetic ring in place (as well as the ratcheted slot). Any loss in efficacy would be minor by not using magnets, I guess. I had come to the same conclusion as you, but thought it was easier to locate a set of magnets than buy, drill and cut rod, as I didn't have the tools to do it accurately. Searching for them, I discovered the issue, hence the original post.