I have the older carbon Shimano 7900 C50 with alum brake surface. It could be worn off but looking to confirm there is a wear indicator hole. The online manual says: "The hollow on the opposite side to the valve hole is an indicator for the amount of rim wear." What-da??? Please in layman terms explain where it is suppose to be. I do have some micrometers so does anyone possibly know the minimum wall thickness??

Thanks a ton!!
Marty