According to Wikipedia, this item can be called a lifebuoy, ring buoy, lifering, lifesaver, life donut, life preserver or lifebelt, as well as a "kisby ring" or "perry buoy".
Plated and enamel cufflinks
Dimensions (Diameter x H): 20mm x 5mm (approx.)
Weight per cufflink: approx. 6.92g (light)
Size = x mm (W x H)