so way up deep in the bottom of the taplet or like on the wall ? or more around the tip part in the middle ?On the bottom side there will be a stamp in mm's, something like 17.035 or 17.018 etc.
Can be a PITA to read. Clean the oil off then with brake cleaner and then wash them with hot water. That lets the # rust slightly in the air and makes it a little easier to read.