Features: Fit for 21mm weaver-picatinny rail mount dovetail rail mount. With hex-key,accurate made. Small size allows for convenient storage and transport. Eliminate canting and improve long-range accuracy with these bolt on scope mounted spirit levels. materials, exquisite technology, fine workmanship, better impact / more durable. These bolts are mounted on the horizontal-surface of the oscilloscope installation to eliminate tilt and misalignment. Specifications: Material:Aluminum Alloy Size:As Picture Shown Type: Cylindrical level(Without wrench) Compatible with 20mm rail Color: black shells + green-liquid Package Includes: 1x Tactical Bubble Level note: 1.Please allow 1-2cm errors due to manual measurement, make sure that you do not mind before you order. 2.Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thanks for your understandings.