Here is a thread that talks about it a little. I don't think it's anything too significant, but it is well known enough for SGC to put it on the slip. I don't believe the stamps are at all factory related. From what I remember, the stamp is usually found on E cards.
http://www.net54baseball.com/showthread.php?t=63296
http://www.net54baseball.com/showthread.php?t=146075