Okay, I removed the slide on my 5.1 to get a look at how the mag catch works. Looking down through the top I can see the mag catch assembly. There's a notch in the catch and when I move the catch I can see a long spring as the catch moves. That screw must go the length of the spring and attach to the other side of the mag catch button. The one on mine is seated really tightly. If you take it out you'll no longer be able to hold a mag but the thing has to come out otherwise you wouldn't be able to put in a new mag catch. It should take a fair number of turns to extract.
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