I don't believe that notch in the bolt carrier extension has anything to do with the gas system. I believe it is to provide a tool for removing the gas piston/puck. If that notch wasn't there, you would not be able to remove the gas puck without making your own custom tool.
The gun runs fine on factory 8rd Saiga-12 mags that were intended for the mag-well equipped Saigas. See below.