Mighty Link chain collar. Variety of sizes available.
 
SIZES: 
Light / Small: 2mm x 14", 16", and 18"
Medium: 2.5mm x 14", 16", 18", 20", and 22"
Heavy / Large: 3mm x 18", 20", 22", 24", and 26"
X-Heavy / X-Large: 4mm x 20", 30"