Altamont makes both sizes of panels. But the small panels are only shown with the SP. They only show the large rubber grip for the GP at their website. And, they have some additional checkering on them on the back. I have always bought the main rubber part from whoever I was buying the hardware from. Midway, Brownells or Ruger. I am 99% certain ALL panels have the ferrules pre installed and Altamont provides the screw, so all you should need is the "grip panel locator" (part no. T03301). I don't know if Altamont stocks that. Midway etc do as well as Ruger. The parts companies always ship so fast I usually go their instead of Ruger. Just make sure they show it as a genuine Ruger part. btw, you might want to check this out. Looks like a good deal to me:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=310942378