The mark on this old bronze letter opener is KB Co, I think? I haven't been able to track down any info about this stamped marking, except to say that this doesn't look like a Keeler Brass piece to me.
This is somewhat crudely made, not what we tend to think of with American Arts and Crafts vintage or German / Austrian pieces of the same period. The hallmark has a sort of an Arts and Crafts style, though.
It's on the short side for a 20th century letter opener, at only 5 3/4" long end to end. It's very tarnished and could probably use a good cleaning.