Since the distinction between the Model 30 and Model 31 by definition is the Model 30 is round butt and the Model 31 is square butt! A simple mis-stamping doesn't change this although there may be more examples out there! Such mis-markings are not particularly rare with S&W.

There is always the possibility that this is originally a Model 31 that has had the grip frame re-shaped and then re-finished. Take the stocks off and post a photo, the difference in the interior contour of the grip frame would settle the question.