it was probably just a hiccup in the casting process, like the first/last of a batch of metal that wasn't poured at proper grade, etc. it happens, and nobody inspects every unit thoroughly as it is made. anywho, it kind of looks like a shifted slug (which would lock the coil up) and, what concers me, is that you discovered this by hearing a screech. if the cone wouldn't move, then the sub would not have made that noise, rather describes the sound an amp can make, played through the speaker. have you checked out the amp? i would make sure there is no dc on the output, and then play a tone and check for good ac/hz to match, and then hook the good sub up, if that passes, to make sure it still plays, and is not damaged. if you get dc, then it can cook the subs relatively quick.