Williams Twilight Aperture TW-3/8"x.050" | 1435 Williams Gun Sight Company Twilight Aperture TW-3/8"x.050" | 1435 Designed For Shooting Under Poor Light Conditions Precision Machined For A Perfect Fit Knurled Grip Makes Changing Peep Apertures Easy (Other Apertures Available Separately) Made In U.S.A.