[size=12]Almost sounds like a "Rapid" teakettle. Griswold made some kettles with bottoms that were a series of concentric grooved circles rather than just a flat piece of aluminum. I guess it was supposed to help it boil faster, but I don't know why. A p/n of 503 would fit into the "Rapid" series, too, but would probably be a fairly small size. I think in their literature, they actually called it "The Rapid," not just "Rapid."
I only have one of these myself; I've gotten pretty lukewarm when it comes to teakettles, since I don't really go for aluminum pieces otherwise. They aren't worth much more than a regular colonial or flat-bottom kettle by Griswold.[/size]