These were discontinued years ago, but apparently Ruger found some laying around somewhere. They show them as "subject to stock on hand" at shopruger.com. The last time I bought one I thought I had paid closer to $50 for the thing. At 15 and change they are a bargain. They aren't as adjustible as the new type mounts, but if you can make your scope fit, they provide a cleaner looking install plus you get to use Ruger steel rings.
http://shopruger.com/22-Target-Pistol-Scope-Base-Adapter-Standard/productinfo/90011/
Here is a better pic of the base mounted on a Mkii
http://www.gunbroker.com/item/586087348
http://shopruger.com/22-Target-Pistol-Scope-Base-Adapter-Standard/productinfo/90011/
Here is a better pic of the base mounted on a Mkii
http://www.gunbroker.com/item/586087348