For 2004 model year, most F-350 SRW optional receivers were rated at 12,500 pounds with a weight-distributing {WD) hitch and 5,000 (or maybe 6,000) pounds with a weight carrying (WC) hitch.
Some very few were rated at 15k/6k. Beginning with 2005 models, the 12.5k receiver was available on SRWs and the 15k receiver was available on DRWs.
The rating will be stamped on the frame of the receiver,