They're actually wear indicators, you will obviously know that the sprockets can be turned to increase the life, that's the reason for the two punch marks. As an added bonus they can also be used as a quick visual reference to check you have both sprockets lined up.
Robin, you don't need a gauge just a visual inspection, if there is a gauge I've never seen one and I've been 1st line all my career, that said I'll check when I'm back in work after Xmas leave.