This is a Kleen-Bore Brush known as the Phosphor Bronze, model A178.
KleenBore Phosphor bronze brushes are precision made for safe" effective removal of fouling and powder residue. Features straight (rather than crimped) wire bristles for more effective cleaning power. The brush remains thick and stout and will not easily bend with force down the bore. The bronze brush is designed specially to not damage the grooved rifling.
Features:
Phosphor Bronze Bristles Designed with rounded bristle tips
Effective and Safe cleaning power
SmallBore #3-48 Threads