Ankle bar, (noun); a bar to which a chain from a metal ring placed around the ankle of a slave, can be secured. One ankle ring is closed on the girl's ankle, the chain passed about the bar and then, on the other ankle, the second ankle ring is closed, securing the slave. [see also: ankle shackles, central bar]

One of the wagons had an ankle bar, and the other one was fitted with the ankle bar from Targo's damaged wagon, which he then abandoned and burned to the grass. 
Book 7, Captive: pg 65