The script on barrel shows that this is a very early Rigby as suggested by low serial number and St James location.It would be part of the first batch of Mauser 98 bolt action rifles. This makes it interesting and shows that Rigby styling cues had not yet been developed.It is still a mystery why the British proof marks are not present. A significant find. I would send pics to Rigby.I am sure they would be interested and maybe give you some info.
The Mauser 98 was introduced in 7mm and 8mm Mauser.(1909 catalogues still show only these two choices). Since the barrel of your rifle has the same serial number as the action and seems to be original it would most likely be one of these two unless the rifle was rechambered. As you probably already know,the .275 Rigby is simply a British loading of the 7x57 cartridge.The 8x57 of that era would be a .318 bore.
As for value, the rear sight alone is worth what you paid for the gun.Well bought.