I believe it stands for Birmingham Nitro Proof.  The BNP proof mark is required by British law before a rifle is released to the civilian market.

Nice carbine JDR!

No hit in the "Serial Numbers of Known U.S. Krags" located in the back of Mallory's book "the Krag Rifle Story".

Your serial number places it between two 1898 rifles, however, your carbine's serial number puts it in the range of serial numbers when 1898 carbines were manufactured!

It's generally accepted that 5000 1898 carbines were produced between serial numbers 113,000 to 139,000 (circa July-September1898) according to Joe Poyer's book "The American Krag Rifle and Carbine".

Your particular carbine appears to be manufactured in September 1898.

Cheers