These are Brass Threaded Inserts from Du-Bro.

Features

Brass threaded inserts.
The perfect way to put threads into wood & mount engines to wood
mounting rails.
Threaded inserts make a simple & effective way to make wing hold
downs.
These can be used where blind nuts are hard to fit.

Includes

(4) Threaded Inserts

Specifications

Thread Size: 4-40
Length: 7mm