Weaver #12 top mount base - Weaver Top Mount Aluminum Base 23

Heads up: The photo on the website is the wrong part. The description is correct and you'll get the correct part if you order the Weaver # 12 base.
If you are mounting a scope on the new Ruger American Rimfire you will need two of these. (They are recommended by Ruger.) I am not thrilled with these bases, but at the moment there is no other choice for Weaver/Picatinny type mount. I would rather have a one piece rail with multiple cross slots so the scope can be moved fore and aft with rings attached to adjust eye relief. Also with a two piece base there is the potential for misalignment between the bases. Using these bases fixes the location of your rings and thus may limit your scope choices.