mikedb,
I have an Interarms Whitworth 7x57 and the same in a a .270 Win sitting in my gun cabinet. I purchased these rifles 25 years ago. They are the fancy versions of the Mark 10. Interarms imported these, the rifles being built by Zavodi CRVENA ZASTAVA, Yugoslavia, according to the User's Manual. It also shows two addresses for Interarms in those days, one in US (Alexandria, VA) and one in England (Manchester). According to the manual, these are Mauser 98s. They have a safety unlike the military 98, but otherwise they field strip and appear to be duplicates of the K98 my buddy owns. I have never had a problem with these two rifles or the .308 Whitworth I used to own. A shooting buddy had one in .375H&H, and it shot well after Interarms replace the first two stocks, as they split. This was time consuming and in light of his experiences I would recommend glass bedding the stock of that .458. Is it worth it? That is up to you, of course, but if that is a Whitworth (marked as such on receiver ring) or Mark X (this on barrels of my rifles), you have a good rifle "Designed and Produced Expressly for Interarms" according to the blurb on the owner's manual.