The top-piece is not the original. Enough has been written to make this certain. The stone is a very good match, but a slight difference in colour shows up in some photographs if the light is at the right angle. 
First, to make sense fully, the words Moshav Marq(os) must have labelled something visible in the same place as being the See of Mark. Setting the throne over the High Altar and over some labelled relic of Mark would have been a start. If the words are to be a label, the model throne would have had to have been over the High Altar underneath a representation of the Christos Pantokrâtôr, so that it symbolised the worldwide Apostolic authority of Mark. The See of Mark, though centred in Alexandria, is the world. (The greater status of Mark over Peter is indisputable. Stealing the throne was the importation of a metaphysical poisoned chalice. The Venetians saw the implications, bought it, and used it to legitimise their independence from the Pope). Second, if the throne is too little for an adult, no-one can ever sit in it, and its symbolism of acts of divine manifestation and judgment, moshavot, would not be lost sight of. Third, there could be an allusion to Mark as the “little child” of the Gospels. This is not Coptic tradition but it is an attested Egyptian view (Clement, Instructor or Tutor, the poem at the end; Origen, commentary on Matthew ad loc. applies it to John but note that the Copts expressly identify Mark and John). The size of the throne is exactly right for a boy of eight to ten.
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dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."