How does one amend a gravestone that has been carved?
Starting from scratch on the backside of the slab is easy, carving into current whitespace is easy and cheap, grinding away the current inscription and carving into the pristine layer below might be a good solution if the stone is thick enough and/or not all of the old inscription needs to be erased.
It look like it was whitespace there, given the height of the letters, but that's a lot of whitespace. I can't find earlier photos of it, though.
Having that much whitespace would make sense if they'd planned to add it later, though it would also be a tell.