I have just taken custodianship of a 275 Rigby - its got the Pall Mall address so late 1960's. Built on a CRF Mauser action, but not a military one - solid side wall, and magazine / trigger guard looks to be a casting rather than machined from solid. What actions where Rigby using in those days - Parker Hale / Brno's or proper mausers?. Trigger looks like a Timney with side safety.
Through that low period they used a variety of actions, often availability and customers budgets were determining factors. I once owned a 416 built on a BRNO 602 action, and stocked with what I percieved to be a pre inletted piece of Fence post trying to pass as Walnut!! This era/period was not Rigbys finest, nor was it for the trade in general. Thankfully Rigby appears to be back on track in 2015.
Best Eric
-------------------- Walk softly and carry a big stick
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.