These are Du-Bro's "Permanent Mount" 4-40 Blind Nuts.
FEATURES: Zinc plated, metal construction. Ideal for mounting engines, landing gear, cowls, hatches, etc. The perfect mounting nut for hard to reach areas.
INCLUDES: (24) Blind Nuts
REQUIRES: Drilling a 4mm hole to install
SPECS: Diameter: 8mm OD of flange Length: 4mm Thread Size: 4-40