Looking at the pictures of that little Webley that Huvius refers to on the Vintage Doubles website;it looks like the rifle may have been through a reproof for said load? 50-200 would be unusual for a .400/.360
Resolution of the photos showing the proof marks arent real good but it looks like a "BP" closer to the action then an "RP" (reproof) in front if that.
Sorry to have added to some thread drift here.......
- Mike