Several manufacturers offer the single gang brass cover single receptacle as an "assembly" that includes a standard metal wall case with plaster ears. IMHO, the box cannot be differentiated from any other wall case of similar manufacturer, it's just that it came in the same package as the cover, and I get to pay additional for it because the manufacturer picked it up and handled it for a bit. Pass & Seymour offers it both ways. See:
Floor Box and scroll down to the bottom the the Adobe .PDF page.
