Don't mistake this rifle for anything Fraser.
It is a Field patent action very likely supplied to H&H and modified by them.
The Field is a very robust, simple design which was used by many makers. H&H made quite a few of them.
It is a great design and this is for sure a quality rifle.
I don't think it has been rebarreled, probably set back and rechambered. There may be obvious gaps around the barrel, especially out on the far half of the forend, this is from setting back a tapered barrel and the forend wood stays where it is of course. Then the forend hanger is moved forward the same amount as the setback. Some guys were better than others at this. Sometimes little slivers of wood are added back into the barrel channel to fill the gap but more often not. Some smiths simply redrill the hole for the forend screw and fill in the old one with a wood plug while others may go as far as moving the barrel lug to keep the screw placement original especially if the screw is set into a fancy metal piece inlet into the wood. My Field has a diamond shaped piece for the screw which is very common.
Make sure you take a good look at the breech end of the barrel if you can to see if it has a sleeved chamber. If so, you are probably stuck with it as a 45/70 and unable to rechamber it to one of the original period British .45 cal cartridges. The 45/70, however, is a great cartridge and easy to find brass for so if you are not bothered by that, run with it. I guarantee it will be a ball!
A chamber cast will tell you a lot as far as seating out the bullet etc. but I have found my various Brit '45s to be rather lenient as far as bullet weights go regardless of the barrel twist.
Also, the Fields patent rifles I have looked at tend to have a pretty big firing pin so I would steer clear of any of the modern high pressure factory smokeless 45/70 ammo.