It would help if you posted some pics. Post war Rigby used FN actions for the .275. British Rigbys are built to very high standards.There are unfortunately many fakes out there. Compare your proof marks and font type with pics of Rigbys on the net. There is a dealer in Canada who recently managed to sell a poorly made replica as an original for $6900. The photocopied "paperwork" had no mention of the serial number. There are some features such as the front sight that are costly to reproduce and fitted to all genuine Rigbys. I have never heard of a cast bottom metal on a .275 Rigby. They used milled steel. If you cannot post pics email me some of the action and front sight Could be a Santa Barbara( used by Parker Hale) which is a lower quality cast action.The Timney trigger would not be original. Rigby keeps meticulous records of all their guns and could verify the serial number for you. The shape of the bolt handle is also a clue.