If the number in the plate were to refer to a gun's number or its position in the vessel, that stylized monogram is not a good match; certainly not that of the dagger's owner, neither the initials of Royal Navy or the like. More plausible would be if they were the initials of the vessel's name ... but still.
On the other hand, if the monogram is that of the dagger's owner, then the digits would refer his number... either in the service ranks or aboard ship, like gunner # 43.
Noteworthy is the fact that the crenellations (per Jim) are only grooved in one of the dagger faces, not all round. Could this be to facilitate the breaking of the dagger's point, after plugging the cannon touch hole, to disable the enemy to take it off ?
... Just fantasies of mine
My guess on the monogram initials is;
A (in bold, as the first), plus a
B and a
N ... or vice versa.