Is this an error, or just the effect of 'large' letters being stamped into a 5 ounce chunk of silver? I had to take this with a handheld microscope. This is the only area on the coin that shows these markings. I didn't see this the first time I looked at the coin, only when tilting it to really take a closer look. I just recieved this coin today (5/4/2011) from the US mint! Thanks Jon
Looks like die damage, maybe from hubbing. That's just poor quality control to let that out on a silver mint set. I could be wrong though.
I would suspect die deterioration. Considering the size of the dies and the high tonnage needed to strike the coins (Multiple strikes?), I would not expect the dies to last real long. Also the size of the coin makes the beginning breakdown of the die more noticeable. Shrink the coin, and that feature, down proportionally to the size of a quarter and it would be inconsequential.