I just received some ASEs from APMEX and was lucky to get a 1991 and a 1992 in a random selection of 6 coins. My question is, the 1991 looks a bit different in the stamping when comparing to a later date, like a 2011 or 2013. The difference I mainly see is in the rays and the edge of the coin. The 1991 looks like it has a radius on the edge and also a thinner edge. Are these things common on a coin that was made a decade earlier? Thanks.
There was a design change or modification in 2008. It actually resulted in the burnished 2008 W reverse of 2007 coins as the mint used a 2007 reverse die with a 2008 obverse die.
There was a slightly redesigned master hub made in 2008 for the obverse. You'll notice the style of lettering is different in "United States of America".
in 2008, the US Mint started making the hubs for the ASEs digitally, rather than reducing them from large engraved models. The fonts and some of the details of the coins were changed slightly, but the overall strike quality improved. Now all new hubs are made digitally.