ankle ring (noun) a metal ring placed around the ankle of a slave to hook a chain too or secure them to a floor ring. 

He also purchased certain articles of slavers' equipment, a display chain, various other sorts of chains, slave bracelets, ankle rings, neck collars, binding fibres, branding irons and whips. 
Book 9, Marauders: pg 166

I was taught to kneel in a certain way, and, when inspected, to lift my head, smile, and utter a certain phrase. Targo, and the guards, made me practice it many times. I later learned that its meaning was "Buy me, Master." In displaying a girl, an ankle ring is placed on her left ankle. This locks on the ankle. There is also a smaller ring, projecting from the larger ring, which also locks. This smaller ring can either be snapped into a particular link in a chain, thus allowing the girls to be spaced at certain intervals, or it can be closed about the chain as a whole, thus permitting the chain to run freely through the ring without injuring or burning the girl's ankle. 
Book 7, Captive: pp 62-63