I just bought my first mec 600 jr 410 and I'm not an expert for sure. I went through all setup and could not find it either but on a small piece of paper in the manual they explained how to install it where primer is installed. Not at home or I would send instructions.
Curly N is correct, of course. Some .410 hulls tend to "stick" on the primer seating stem that pushes the hull down onto the primer. The clip in the photo, helps to hold the hull down when the seating stem is raised. -Ed