You're looking at a brand new, 2 inch by 4.25 Glass Magnifying Lens (Cheater Lens). Prior to hitting the "Add to Cart" button, simply choose your desired magnification. This Mag Lens fits most of the popular helmets from Jackson, Huntsman and Fibremtal, regardless of the lens size in the helmet.
These cheater lens' will Not work with 3M Helmets or Lincoln Viking Helmets, however the Miller brand cheaters will work and can be found here.
MPN: NAS101MP - Cheater Lens