I'm looking at a Super receiver without the stock on it and it looks like there are two places for your problem to occur. Without getting to details (which I couldn't discuss anyhow), my bet is that trash or lack of lubricant in the inertia block is the problem. With the safety on, the block is allowed to move forward under spring pressure. With the safety on, it's pulled and held back. My bet is that trash has gotten into the slot on the top of the block or into the pivot point on top of the trigger. Either case could prevent the block from moving forward and allow the gun to fire. My bet is that a good cleaning and light oiling will solve the problem easily. Take the stock off, clean the block with solvent, blow the crud out with compressed air, apply some oil, blow again to remove the excess oil and get it into all the crevases, and you should be good to go. Hope this works. If not, it may be that the spring that pushes the block forward is weak or broken. But I doubt that's the problem.
