Galen, did you mix up "naked core" and "coreless"? When I first saw the question I was thinking the same as you but now that ZZ has posted I'm like "oh right!"
In Collecting Antique Marbles, Baumann allows for ribbon marbles to have either one or two ribbons. (He says it's debatable with 2 ribbons would count as a double ribbon or a divided core.)
He also seems to apply the term "naked" only to the ribbon style marble - when there are no outer bands, like ZZ says. (So then would Baumann apply the word naked to a double ribbon?)
And then the divided core has more parts inside. Baumann says the production style of the divided core is similar to that of the ribbon core. But a divided core is still considered a different type from a ribbon core.