Based on the Bulgarian style center support these DO NOT HAVE THE FLAT on them. Other countries have a larger diameter center support with the flat cut on it for magazine clearance. Most uninformed people using the older flats with the center support holes too high thought that the flat was for bolt clearance.
The center support rivet was copied from an actual Bulgarian rivet and lengthened by .5mm.
DO NOT USE THIS RIVET IN A DIFFERENT CENTER SUPPORT SLEEVE THAN WHAT COMES IN THE SET! The rivet size is matched to the hole size in the sleeve. If you use another sleeve with a larger hole the rivet will expand and try to fill the larger hole and won't form a proper head.