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: (4) Blind Nuts

REQUIRES: Drilling a 4mm hole to install

SPECS: Diameter: 8mm OD of flange
Length: 4mm
Thread Size: 4-40