I also have never seen or heard of an original .416 Rem chambering in an Interarms Whitworth rifle. Latter production of these rifles had the chamber marking rather distinctive in Cursive (script) writing and quite handsomely accomplished. Located on barrel left, chamber area. If the presumed "416" marking is such a style & location matching, perhaps a real inference. But I'd be surprised!
Best & Happy Holidays
John
