Don't be surprised if that pin beside the screw has to come out also. I have a standard STG parts kit that I just tried to compare, and the rear sight I have is not like what you have. I'm not familiar with how the para/folding stock is held in and whether or not that pin is for the folder adapter only or connected to the sight. Regardless, the sight will probably be snug in it's dovetail, so a couple gentle taps with a brass punch might be required to get it out after the screws (and maybe pin) are removed.
