Trumpet Embouchure Training Practice Mouth Piece

These are commonly used by brass players to practice and perfect proper embouchure. Usually an actual mouth piece is cut to create what you see here.
The idea is that you can see if you have the correct placement of you lips on the mouth piece. Then when placed in the instrument you can practice blowing straight ahead. When attached to the instrument you can hear if the air is being forced into the instrument or escaping through the openings of the mouth piece.
This is for a Bach trumpet