So I just took delivery of the DeSantis Storm Packer which is a stock sleeve to hold two extra M9 mags. I used to have a Ruger PC4 before I sold it and when I had it I wished I had bought the Uncle Mikes stock sleeve for it. When I finally went to buy it, you couldn't find them, so I know things like this are scarce because they are for a specialized weapon. That's why I just purchased the Packer.
First impressions are that it is well built and fits on the stock very well. What I can't believe is that they used snaps to close the pouches instead of Velcro which you could adjust according to the length of the magazine. The pic you see has two standard 15 round mags in it and look at all the wasted room, that could be tightened down if they closed with Velcro. Stupid.
(See post below, issue fixed)
Having said all that, I absolutely love my Beretta Storm Cx4 in 9mm!