You shouldn't have had to remove anything from the retainer. The roll pin should provide just enough tension to snap into the notches on the "screw" head. It's not really a screw. The opposite end is peened to prevent it from coming out, and there are no threads on it.
The "screw" doesn't come out of the handguard retainer. When you turn the "screw" head, the shaft turns just like a standard AK retainer. When the shaft lines up correctly with the notch in the barrel, the retainer is locked in place. Turned 180 degrees, the shaft will disengage from the notch in the barrel and the retainer can be pushed or pulled forward.
Since you don't know whether the "screw" is in the lock or unlock position now, you will just have to try to pull the retainer forward with the "screw" in both positions. Be careful not to pry against the handguards or you may damage them.
Additional questions:
1. Any rivet set that will work on an underfolder or side folder will work just fine. Don't use the Tapco rivets for an AK build.
2. The gas piston retainer pin is about 3/16" to 1/4" back from the front of the carrier. The ends are peened over and ground flush with the rest of the carrier. Use a bright light and look at the sides of the carrier (3:00 and 9:00 positions) at an angle, and you may be able to see them. You can also try a little WD-40 or mineral spirits down the recoil spring hole of the carrier. The oil should wick around the edges of the pin and outline them.
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