Welcome to the forum,
Yes you would think that having Sheffield stamped on the pot would mean it was made there...afraid not. Welcome to the wonderful world of silver marking.
Your pot was made by the US company of E & J Bass, New York. see here:http://www.925-1000.com/silverplate_B.html
Why the Sheffield mark? The pot could be made using the Sheffield plate process which consists of a copper sheet sandwiched between two sheets of silver but this is not very likely. A number of US companies stamped the name Sheffield purely as a marketing tool because Sheffield was renowned for good quality silver and plate.
Date wise you will note that the company was in business from 1890 to circa 1930.