Not really anything to do with curiosity regarding SG per say. I think anyone whose had a high school Science/Chemistry class is probably familiar with it. My curiosity is the exact process you utilized as the results you obtained especially some with such extreme variance compared to mint specifications seems off. I kind of suspected you used the suspending the coin method which is inferior in my opinion to using the scale, distilled water and graduated cylinder method. Too bad the coins are not all still together as I think if you retested using the method I mentioned it might give different results all together.
Note the information posted on the other thread "A Compositional Analysis..." in which the temperature and water are mentioned. Indeed, these are important items, as are trapped air pockets, et certera. Gary in Washington
I still have the silver coins I tested!! Gary in Washington (even the Egyptian one, and the .100 Mexican Peso).