From tech-mavic.com site
“INTERNAL: Process: The inside of the braking surface of the rim is machined on both of the wings of the rim. Principle: When there is too much wear on the rim, a little hole appears on each of the 2 braking surfaces of the rim. Depending on the adjustment of the brake pads, it is possible for the wear indicator to appear on only one of the 2 braking surfaces. In any case, the appearance of the wear indicator on at least one of the 2 braking surfaces means that the sidewalls are too thin, and it could be dangerous to continue to use the rim. It should be replaced as soon as possible. The position of the wear indicator is marked by 2 yellow arrows on the stickers on the rim, opposite the valve hole. Refer to the chart on the previous page to find out which rims offer this internal wear indicator.”
Not 100% sure this applies to Open Pro as I don’t have the chart it refers to.
Am in same boat, v.worn rims after 4 winters of grimy brakes.