That's got to be worthy of some further research - whatever it is - it wasn't Bubba.

Certainly not one of the classic 476XXX gallery rifles, but I would NOT rule out Butler's guess. Another possibility might be some sort of clearance cut for a sub-caliber mounting. I don't see that extra bolt piece as any part of a sight mounted to the rifle however - but perhaps a way to re-cock it remotely - via a wire? - if the breech were un-accessible? Just another thought.

Got to be more careful what I post!