This is a Swivel Ball Link set by Dubro.
This set is designed for use with 4-40 threaded rods.

FEATURES: Black plastic construction. Brass swivel ball. Full hardware package.

INCLUDES: Two Ball links with inserted brass swivels. Two 2.2x13mm hex head screws. Two 5x3x2.5mm brass spacers Two Silver 2.2 hex nuts.

REQUIRES: 4-40 rods Assembly onto kit.

SPECS: Length: 25.2mm (.994″) Inside diameter of swivel: 2.8mm (.112″)

COMMENTS: The screws used to mount the link are 2-56 size.