To me it looks like a typical G.Gibbs, Bristol, sporterized Mannlicher, once destined for resale by another. Gibbs then imported lots of Mannlichers and Mannlicher - Schoenauers from the Steyr military production then, sporterized them and sold many with their own address. But they also supplied many other gunmakers with such rifles. I have seen such Gibbs - Mannlichers marked by Atkin, Rigby, Purdey and many more. Some retailers apparently did not care to mark these rifles themselves.Early ones were still marked 6.5mm, until the Brits invented their own name for the cartridge, .256. If there is a B - prefix serial number, it is a Gibbs. What is the date on the left receiver wall? If I knew the date and the serial number,I could verify the G.Gibbs provenance.