I recently bought a used/new Ruger/Marlin 1895 SBL in 45-70 at my friend's pawn shop.  I removed the rear sight to install scope rings and noticed the screw that holds in the rear sight also holds down the rail in the rear.  I mounted a scope ring and saw I could lift the rail a little without the rear sight and screw installed.  Does anyone know if there is a small screw to install to hold down the rear of the rail or will I have to use high scope rings and leave the rear sight installed.  Hopes this makes sense.  Thanks for all the hlep.

Take the old screw to a real hardware store and buy a new one of the proper length.

On JM and Remington guns the scope base screws are an oddball thread pitch that you are unlikely to find anywhere except for marlin mounts.  Unsure if ruger uses the same threads for the scope bases.  Be careful putting them in - the receiver above the bolt is thin and it’s easy to strip the threads.  

If you don’t need the extra length of the XS rail you can get a cheap scope base to replace it.  
If you need the length then find a screw.  

Another option could be to remove the peep from the base and reinstall the base without the peep.