The Ruger Mark II, III, and IV rimfire pistol series comes standard with three 4-48 threaded locations for optics mounting. Factory models come either with picatinny rails or with threaded plugs if no rail is included from the factory.
Some models include two threaded locations under the barrel for an under barrel picatinny rail. These models include the Ruger Mark IV Tactical and the Mark IV 22/45 Tactical.
All threaded locations are blind with the exception of the rearmost threaded hole. For the Mark IV series these blind holes have a threaded depth of approximately .15 inches. The rearmost threaded location has a thread length of approximately .125 inches.
It is important to note that if a screw is too long it can interfere with the operation of the bolt. Screws used for these holes should no longer than 3/16 of an inch for most applications.