I put the HP double mag holder on my 995TS, it works perfectly and, easily when mounted properly. (The bolt replaces the bolt at the bottom of the second (longest) rib in the stock.)
When installing it, the narrow ears of the clamp are set toward the bottom of the stock and the bolt is tightened very snug and should be set with lock-tite.
The stock 10 round mags fit very nicely with the base of the mag parallel to the butt end.
To remove the mag, grasp the bottom edge of the mag and give it a firm twist outward. [i]The 'bottom edge' is the spine of the mag.[i]
The HP mag clip will hold the Pro-Mag 15 rounders but, I make no promise that they will stay secured when firing. I haven't tried it in the field yet
Here are some images to show how it looks.
I hope that will answer every's questions on this HP accessory part.