Fit over the front of the K40 mirror you are working with (mirror 1 and 2 - there is another print on it's way for mirror 3). This is to help when you align the K40 laser mirrors. You will need to cut a piece of metal from a soft drink can (or a soup, baked beans ect. can lid), approx 24x32 mm. This gets inserted as shown by the blue shield in the first image (there will be 2 small triangles sticking out the bottom, bend them over so the shield stays in the unit - do not glue the shield into place as that may hinder pushing a piece of paper in). IN FRONT of this (i.e. where the laser will hit), insert a piece of paper cut to the same size. The shield is there to protect you mirror from smoke and debris when shooting dots on paper to align the K40 mirrors. And it is easy to remove and refit a new piece.