"JDR" - Welcome to the KCA Forum.

I see nothing that questions the 'validity' of your U. S. model 1898 .30 cal. magazine carbine. Your stock and hand-guard were an update to make your carbine identical to the model 1899 carbine.

There may be a 'circled script P' on the stock behind the trigger-guard. This indicates 'function and proof' at the armory. (There is probably no 'cartouche' on the left side of the stock wrist area).

Your wood was probably put on when your rear-sight was changed to a model 1901 'Buffington' style sight. Originally, your carbine would have had a model 1896 carbine sight.

(Initially, model 1899 carbines used the 'rifle style' hand-guard, like yours. Later, the hand-guard was changed to a style with a 'hump' to protect the rear-sight).

I have never seen a Krag before with a Birmingham Nitro Proof. I imagine your Krag may have been imported to G.B. after WW2 ... or maybe, it was an American Donation (of sporting arms and optics) during the desperate days post-Dunkirk, when Britain was fighting the Axis alone! (Maybe you're suppose to send it back  ;D).

The Marble's front sight blade was a popular addition to Krag rifles and carbines in the U. S. to better adapt them for Hunting. The Marble's company was located in Gladstone, Michigan, (Northern Peninsula of my State). They were famous for Knives, Axes, gun-sights, and other hunting gear.