The "A" and "B" lenses are the same except for the distance to the screen, with these attachments you can "boost" the height of your "B" lenses so that they work like "A"s.
Meaning, if you use A lenses and they are damaged, you can replace them with your B's + the attachment, Alternatively, even if your "A"s aren't damaged, you could use these when demoing to groups to preserve your "A" lenses.
For Instructions and more information go to the about section on the main shop page: https://www.shapeways.com/shops/vrgear