"This Piece Of Brass And Possibly Lead Is Embedded In A Piece Of Planed Wood. It Has Been Sitting On A Shelf For Years And I Have No Idea What It May Have Been"

Answer: "That's a bullet. Someone shot it into a tree and cut it in half. The rough part of the wood is the bullet path."
Pretty sure you are wrong.... the black/blue pages in those are thin. photo looks more like an album.
Better... er... what? That would depend on the specific function of the slide rule. I used these in an engineering drawing office early 70's
#56... That is what is left after removing a metal screen that was painted at one time
came with firearm accesories... Probably a wick for old gun.
But the two things can look more or less identical.