Bioflex barbells (retainers).
Available in various sizes:
  
1.2x 6mm, 8mm, 10mm (with 3mm balls)
  
1.6x 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, 22mm, 30mm (with 5mm balls)
Spare balls are also available.
Sold individually.