If the frame is stripped, perhaps the seller honestly is confused about its origin and model. Weigh it. A size 58 Reynolds frame ought to come in under four pounds. The fork should come in under two pounds. Say around 5 1/2 pounds for the complete frame and fork in size 58. If the combined frame and fork come in at a total of 7 pounds or 8 pounds, well, that ain't 531.
The "seat post" test makes sense. The cheaper tubes had thicker walls, and required seat posts that were smaller in diameter than the Reynolds 531 frames. And, with precision calipers, you could measure the thickness of the seat tube and compare that with the factory "spec" for Reynolds 531.