Mk II was not drilled and tapped and did not have a base.
The MK III models were d&t from the factory and did include a one piece Weaver style base, maybe even Weaver brand, I'd have to look.
I suppose Ruger got calls and letters asking if they'd start including this or that specific (expensive) base instead of the one they did - folks tend to figure the factories are happy to cater to their individual whims - so they probably figured the heck with it, let folks spend their own money.
Given the nice things they added to the Mk IV I wouldn't give Ruger too much of a bad time about the lack of a base on the Mk IV, last I looked a one piece Ruger base was about 10 to maybe 15 bucks, a lot less than the cost of the second magazine.
