Countersunk half hex rivet nuts have a visible 90-degree countersunk and require a pre-drilled countersunk hole to enable them to fit flush; they are inserted into a hexagon hole which eliminates the potential problem of the nut turning in the hole after installation.
These rivet nuts can also be called countersunk bolts, but please note this style should not be confused with reduced countersunk head rivet nuts which have only a slight countersink.