Book of Kells

Reading about the great masterpiece of what I have learned is called "insular art"  (i.e. art of the British Isles in the early medieval period) -- and looking at a site where the entire thing can be looked at page by page -- 

DRIS Trinity College Library Dublin

-- I found myself fascinated by this one (even set it as my home screen picture):  It's the incipit or title-page of the Gospel of John.  It shows the evangelist, holding up what appears to be his book, which you are about to read, but which he is in -- a nice metafictional touch.  But odder is that at the bottom of the page there is a tiny pair of feet, just like the Saint's; and on the left and right, two tiny hands, gripping; and above -- though the page has perhaps been worn away or wasn't completely copied -- what looks like a coat and tie and a faint nimbus above it.  Who is that and why is he standing behind the saint?