This is one of those "modified" chips that may have been done by the casino. No one I know has the answer and this chip has been discussed for at least 10 years. It's a very good job, and I tend to believe these were done intentionally for the casino, maybe by the manufacturer.

If you are able to modify one of the solid color ones, I'd be interested in seeing the result. It looks to me to be a difficult task; these were probably not done with a felt marker.