This is a Leupold Rifleman 1-piece base commonly used to mount optics on rifles like the Ruger 10/22. 

• Construction: It is machined from anodized aluminum which provides a lightweight yet durable platform for scopes.
• Compatibility: This base features a Weaver-style slot design making it compatible with Weaver rings as well as Leupold Rifleman rings.
• Finish: The mount has a matte black finish designed to match most modern rifle receivers