As above - originally a "United States Rifle Model of 1917" - aka M17, M1917, P17. Then, re-worked by BSA in England sometime after WWII. With that original shape bolt handle and the original "bent" tongue on the trigger guard, would have been one of the lower grades of BSA conversions - a Model A or Model B - I do not know how to tell difference. Higher grades got straightened bolt handles, straightened trigger guards, new sporter stocks (lower grades had sawed off and re-shaped military stocks), and I think the highest grades got new, not military barrels. Rear scope base is Weaver #36 which has .605" hole spacing - the only base in the Weaver line-up with that hole spacing, which BSA chose to use.
All 98 Mausers that I have worked with are "#### on open" - firing pin gets cocked as the bolt is opened. On these, like the pre-98 Mausers, Lee Enfield's and P14's, the action is "#### on close" - some find it un-nerving that "cocking" the firing pin is part of the process of chambering a round and closing the bolt. They work very slickly, once you are used to it. Other "advantages" over original 98 mauser include helical breeching, which can be important with dirty, bent or over-pressure cartridges - both for chambering as well as extracting.
As far as value, I would agree with saskbooknut - $150 to $200 - possibly $50 more if it had a stock, but with that bent trigger guard, would probably have been a cut down mil-surp stock - most "sporter" stocks from BSA seemed to use the straightened trigger guard. Action is way too far gone for a military re-build - but would be a very tough and rugged hunting rifle - my Dad carried similar to that one since 1948.
