@robhic;
I did in fact go over the rest of the die with a magnifying glass and found nothing foreign in it, or any other damage, for that matter.
@Unclenick;
Your suggestion took me off-guard for a minute. Closer inspection of the pictures I took shows a very regular, perfect crescent-shape right at the edge of the sleeve, so I suppose it may be possible that it's there for a reason. However, when I disassembled two other seater dies, neither showed the same crescent on their respective sleeves.
@243winxb;
It is a Hornady bullet guide sleeve (you win a cigar!). And I probably will replace it. I was just wondering if anyone else had experienced something similar with their bullet sleeves, and if this kind of damage could in some way cause improper/unsafe bullet seating. I don't see how it could, but I've been wrong about such things often enough that I thought I'd ask.