That's totally the original rivet... Looks like they tried to screw it out or back in and since the rivets are mushroomed to hold them in, the ad-hoc slot means they twisted it in with force and the bugled portion threaded itself going back in.
ANY screw has concentric threads... even hand tapped ones. Those look to ME just like what I described above. The Original rivet... modified to be put back in.
ANY screw has concentric threads... even hand tapped ones. Those look to ME just like what I described above. The Original rivet... modified to be put back in.