One option is made for standard 1 inch bars.
The other option is made for 1.9 inch diameter SPECIALTY bars that accommodate Olympic weight plates. It will NOT fit onto the rotating shaft of a standard Olympic barbell. It will fit all the Mercury and Thor bars sold on this website that are made for Olympic plates. However, both options are sold as a single collar, not a pair, for use on a ShinBlaster by Mercury & Thor Fitness Products. The Shinblaster only requires one collar
Specialty bars generally have static (non-rotating) ends onto which the Olympic plates are loaded; this is due to the standard steel tubing sizes that are available as raw materials. Standard Olympic barbells with rotating ends are MACHINED, and have slightly wider diameters than specialty bars.