RHC
You are right about removing the cap on the end of the mag tube.If the slot in the screw is messed up, I would first order a new screw (www.brownells.com). Once you have the new screw you can take a motorized "dremil" tool with a small ball cutter and carefully cut a deeper slot in the screw. Before you attempt to remove the screw, soak it with penatrating oil and let it sit for a while. If the threads are stripped, soak as before and try to get a very small flat blade screwdriver under the edge of the screw and lift up on the screw while turning the screw with a second screwdriver using light downword pressure.
I cannot remember the capacity of the magazine right off hand, but cannot belive that anyone would have put a plug in it. Good Luck John K
One other trick for messed up slots and tight screws,. If you have a drill press, secure the part in the drill press vise (be sure to put something protective around the bluing and don't crush the part) then put the correct size screwdriver bit in the chuck. Center the screw under the bit and lower the chuck and bit into the screw slot. Hold the bit down on the screw with a fair amount of pressure with the handle and manually turn the chuck with a wrench on the flat sides of the bit. It is a good idea to unplug the motor just in case. This seems to work when nothing else will and keeps you from marring the part from slippage of the screwdriver.