I brought this back from Korea. I briefly used a captured DP and kept this pan magazine. Because it was used by the US, this is likely one of the rarest finds yet. Not really a find, but something I've been holding onto for 60 years! It gave me a hell of a lot of trouble reloading it, but it gave good service.
If memory serves, the Brits didn't use our .50 BMG much. It's likely .55 Boys. The top is a fluted training round. If the site you found it on was an armor training ground, however, it could very well be .50 BMG.