Got one for my S-12 yesterday off GunBroker. Great upgrade! However I run into a serious issue - can't use any mag except the 2-shots one. I believe that's caused by the mag lock, which is mandatory in CA. I've got a bulky aluminum block one preinstalled by the dealer (don't know the actual origin of it, possibly - Izhmash itself).
Anyone had that issue before? The only way to fix that, I could imagine so far, is to file an arch shaped guide in the mag lock, hoping it will be enough to shift the top front of the mag just about 3/16" backwards required to free up the front notch of it enough before rocking the mag in.
By the way, I have a 3D printer at home, and plan to make a proper butt for the stock with it to cover tangs. I'm going to model it an inch lower down as well, so it could sit higher over the shoulder. Anyone interested? Let me know, I'll open a separate thread and would consider any suggestions on the final design. So far I came out with something like this: