I agree with Dan, the screw was stripped and the hole retapped to a larger size.
This small screw is only for holding the top lever in place so that is does not ride above the top lever spring and then become very hard to impossible to open the gun.
This screw pivots with the top lever and can work it's way loose sometimes and if you see this tighten it up. The head of this screw fits into a counterbored hole that allows for the pivoting. Using another screw that does not fit into the counterbore will ultimately come loose.