If in fact, this isn't an "Original Optimus", and that it has been upgrades/enhanced, what would be a fair price for this shotgun?

In my opinion it is still a very attractive gun, and I would enjoy hunting with it, even knowing its artifical origin and checkered past. A talented engraver spent many hours on this "upgrade." That alone might cost $5k or $10K.

Is there a general guideline: that it would now be worth 10% or 20% of a true original?