How fascinating!
Is it just the photo or does the inscription look amateurishly carved? How is the 'box' attached to the outside of the stone? Iis it recessed? Is the 'box' marble ? I wonder if it was purely decorative with nothing behind it.
I wonder if the church holds any records of burials and descriptions of stones? May be some clues there.
I know it would not be right and I would never do it, but if it were my ancestor the temptation to have a little poke around would be very great.

Added: Looking again, is the box hollow do you think? Whereabouts is James buried?