I am not sure who this John Powys is. The clues are that the style is similar to that of Thomas Jelf Powys' and that the crest is the same with a vertical spear, in contrast to the angled spear of the desendants of the first known Powys armiger, Thomas Powys of Henley. These things are a little more obvious with the information that a John Powys was the uncle of Thomas Jelf; this John was the eldest son, inherited Berwyke or Berwick House in Shropshire, had one son, Thomas, who dsp and then left Berwick to his cousin Thomas Jelf. So I am guessing that this plate is that of John Powys, owner of Berwick and who died in June 1753.