I am learning grading...still getting to a solid foundation. Since I have been collecting the State Quarters like almost everyone, condition is everything. I like raw coins and albums versus TPG slabs. So my question is this: Once you get to MS67 or more like MS68, is there any visible difference to MS69/MS70 or is it a crap shoot through the third party graders at this level. I have many coins that appear better than my TPG MS67 reference slabs used to help me understand. I have looked at many coins graded 68-70 and I have trouble telling them apart in the majority of cases. How is it done at the TPG...do they blow up with microscope to the point of seeing every possible dink, and if so, how can they distinguish a genuine dink from a die maker dink...the die makers are not perfect, or are they??
TPGs rarely use a loupe when grading a coin, much less a microscope. If I am understanding you correctly, a 'dink' on a die would would be a small indentation caused by a hit or a scratch. That indentation would cause a raised element on the coin. After a coin has been struck if it is hit or scratched the 'dink' will be below the surface of the coin.
A 70 graded coin should appear flawless under a 5X glass. A 69 might have a tiny imperfection at 5X but no more than 1 or 2 miniscule flaws.
Technically, the answer to your question is no detectable differences, at least for a 69/70. From PCGS; So, if you are using just your eyes, you should be able to find yiny specks on the 68's, but nothing on 69's or 70's. And nothing on the 70's even with a 5X loupe.
