There were a bunch of companies selling those converted revolvers, but most were buying them from Interarmco. I think Winfield may have been importing them and possibly having the work done here, but if so that was uncommon. English labor was cheap at that time, not to mention of high quality, so having work like that done in the States wouldn't pay.
The easy way to tell is to look at the proof marks. If they indicate .45ACP, then the work was done in England and the guns proved after conversion, as required by English law. (The law required that guns being sold out of government stores that had not been proven in a way acceptable under the law - which meant any U.S. guns - be proved before commercial sale or export. But if the guns were to be converted, the work could be done first, then the gun proved for the new cartridge.)