Some where there will be some sort of marking system. When ever we get another Vauxhall recall or rework we're given the info for that specific componant to check as to whether or not its in production range.....and its the same for all manufacturers. As an example, we've got a steering intermidiate shaft problem on the Corsa D's...but there's only sommot like five out there to be taken as faulty...just five out of all of them! If they didnt stamp batch numbers on them all, then we'd have to change them all lol