"What proofs does it have?"

Did I mention that I am a novice at best when it comes to Doubles? I think another picture would help out here!

"How do you know it is an Egg?"

It had "Henry W. Egg" engraved in 2 or 3 places on the rifle

"Why do you think it is a 450/400?"

Just a guess. What would be some other possibilities in .40 cal?