I believe brass knuckles are prohibited weapons in Canada. Not sure in the States.
Pretty sure they are illegal in most states as well.

In LA, vendors can get around this by wrapping them individually in a package labeled 'paper weight.' Problem is, if you are caught with them unmarked they are
brass knuckles in the eyes of whoever finds them, making carrying them more trouble than they are worth.
Having them hanging behind the counter in a pizza place seems pretty dumb, though not really a big deal.