DUDE!!!!
A bonafide John Rigby Double is worth a FORTUNE!!! In 470 Nitro Express? Holy Cow what a great rifle! I'm drooling!
It is almost certainly worth at least FIVE FIGURES probably way up there in those five as well.
Don't let anybody swindle you out of that piece, it's about as irreplaceable as they come.
Please post pictures. That's an awesome gun!!!! Seriously, you need a safe to put that thing in.