I Have put my hands on a Gibbs double in .458win. The rifle was originally chambered i .450Nitro express but later in the 1970s chambersleeved by Rigby`s into .458win. However I have checked and consulted with a gunsmith( a real one) that it is no problem to recut the chamber back to .450 Nitro.
The .450 Nitro express reamer will clean-up the chamber fine just with a finisher-reamer.
It looks to be a fine gun in overall condition.