The "throne of Mark" is the patriarchal throne of the Coptic popes. This web page has a number of photos of the throne. It is clearly a representation of the cherubim throne from the vision of Ezekiel, with a lion, bull, eagle and man panel, and many other temple motifs. Artistically it seems to date from the early medieval period of Coptic art.
Stephan Huller's book, The Real Messiah is a highly speculative interpretation of the this throne as a proto-messianic coded symbol.