The standard Ruger 10/22 Reciever has four threaded holes on the top of the reciever designed to mount optic rails for scopes, red dot sights, or other accessories. These threaded holes are spaced out unevenly and threaded in #6-48 which is a very unique thread type only used in optic mounting for the most part. Please see the drawing below for dimensions.
For more information feel free to download this information package.