I'm thinking that BT would be the better place to call.
BT Paintball Designs, Inc. - 1-800-724-6822
I know there's no serial number on the Rip Clip itself, the only other place I could think of looking would be on the original box. (Usually near the barcode.) It's quite possible that the warranty card is generic, and the reason it has a spot for a serial # is because it's sent out with the guns as well, in which case I'd just leave it blank.