hi all i agree with paul murphy best place to start would be mint records or die manufacture records these may have been outsourced to different die companys.the mint may have upgraded their machinery giving slightly different results to their pressings over a long period of medal production,also just because the machines are the same it does not mean the the finished result is the same, this may be true of dies too .i work in the high quality print trade and have seen 6 printing machines of the same manufacture give 6 different print results this was very noticable in the old letterpress days which worked very much like die presses eg pressure and lead type much like a die.As for rivets etc this may still be a fairly reliable way of dating medals it seems to be a natural industrial progression start with rivets progress to screws then do away with the lot,its all a question of cheaper production and lower cost.phew, good luck richard