US Optics Ext Cant Indicators with Swivel Bubble Level available for Picatinney or Weaver (Rail Mounted Style) and 30mm Rings (Ring Mounted). Screw on cover protects glass vial from damage when not in use. Those Riflescope Ext Cant Indicators by U.S.Optics include cover. USO Rail Mounted Swivel Bubble Level Ext Cant Indicator (Contoured) attaches to your Picatinny Base. U.S. Optics Ring Mounted Swivel Bubble Level Ext Cant Indicator attaches to your 30mm scope tube. Screw on cover of those USOptics Ext Cant Indicators protects the glass vial from damage when not in use. Level folds away 90 degrees when not in use. Made from 6061-T6 and Type III Hard Black Anodized.