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.