I was lucky enough to see this work as it developed.
A new, upscale home was to have a receptacle placed in the living room floor. The contractor used an ordinary flanged plastic box, a clock receptacle, and then (amazing!) covered it with a proper floor cover.
Since the box is not adjustable, there is a gap caused by the thickness of the hardwood flooring.
The irony of this clever ruse -which slipped past the inspector- is that the brass cover plate cost quite a bit more than a proper plastic box / cover assembly!