Cooper is now drilling and tapping their new rifles for the larger 8-40 receiver screws...  Date of this update notice May 24, 2018

Low .400 inch, Medium .500 inch, High .600 inch - as measured from the contact point at the receiver to the bottom of the ring cradle.

The extended front ring brings the front ring 1/4" closer to the rear ring than the  non-extended version.  This extended option is typically used for rifles with a longer action and / or a scope with a shorter tube.