This is a hard plastic firewall grommet for oversized 4g to 1/0 g sized wires. It has a ip68 rating to help keep water that may splash onto the wire from entering the cab. 
 
12-16mm inner diameter
IP68

This grommet DOES NOT fit my 1/0 FAM cable. For that cable you will need a larger grommet.
This grommet DOES fit my 4awg #FAM cable with flex loom.
This grommet DOES fit other car audio brand 1/0 cable.