The magazine button I'm referring to, is the one on the side of the actual mazazine itself, not the magazine release lever on the pistol's frame. All Mark II's and I's have the magazine release underneath the grip, unless they've been aftermarket modified by someone like Broken Gun Ranch.
The magazine button I'm referring to is the one that is part of the follower inside the magazine (clip), it's the button you place your thumb on to load in the cartridges. The side of the magazine where this button is located is one of the characteristics that helps determine a Mark I from a Mark II.