CJF, you have a most exquisite rifle there mate, an heirloom for sure!
The engraving alone just quietly states quality, nothing less.
Glad you had success straight up with the 300gn jacketed NFB load, I guess the next thing to do is do a cerrosafe chamber & slug the barrel to prove your theory, I'm in agreeance with you, I believe it will be a tapering bore.

If you can chamber a .458 projectile okay then you should have sufficient chambers to release the bullet, I'd only use lead projectiles rather than jacketed to swage down the bore - others may prove me wrong here.
I'm rather conservative when coming to doubles!
Looking forward to your next range report, enjoy your old girl & take her for another spin!