Sorry, I might have the wrong idea here, but are you talking about the groove around the circumference in the centre of the rotor?
If so, mine's exactly the same (I saw no indication it was from undue damage) & I was under the (possibly mistaken) impression it was something from the production process - maybe something to do with balancing?
I think I remember Dave500 posting about this at some stage, but it's rather difficult to revive some of the past threads these days, even with Google advanced search.
Maybe worth checking with some other 550 owners to see if it's a sometimes a normal occurrence?
Edit :
http://forums.sohc4.net/index.php?topic=133777.0Mostly talking about the butt joint, but you can also see the gouge present.