I would have to check but I believe that Dale only had a scan of the IO-OZ with natural rim. Many chips in Dale's book are based on scans of chips owned by others. There are many examples of chips with natural rims shown in Dale's book that turns out that the rims actually had a color but was worn off.

Also a natural rim chip is usually carved differently than the same chip with color. Sometimes an extra ring is added to the nutural colored chip. If you note in the book, there is a big wide blank rim just begging for color. I would say that that chip originally had color. Come to think of it, I don't know if any $5 chips were originally natural. Without checking, I believe that every $5 chip came with color,. IO-OZ is not an exception