I have an older slabside.  It came with a smalll scope mount supplied by Ruger.  Some time ago Ruger also offered a weaver style base with a few slots (3 or4?).  This base fits the factory drilled and tapped holes.   I bought one of these from Ruger parts and accessories dept.  I think Ruger now includes this base with some of their .22 auto pistols.  With the limited number of slots, I had some trouble getting the red dot I had to fit, not enough spacing options to fit the built in weaver on the red dot.  I suggest getting an after market base with as many slots as posible.