Brass compression sleeve. OD (outside diameter) tube size. Sold in bags and also in bulk. Manufactured to include no more than 0.25\% weighted average lead content on wetted surfaces.
3/8 Black Merchant Coupling
6pk 3/8 Release Clamp
3/8x3/8FPT Barb Insert Brass
1/4x3/8MPT Barb Insert Brass