Concise Bible Dictionary:

Various words are used for these instruments of punishment.

From Manners and Customs of the Bible:

Some would understand by ξυλον “stocks,” simply a bar of wood to which the feet of the prisoner were chained. Others suppose the instrument to have corresponded to the modern stocks, consisting of a frame of wood in which the two feet, separated far apart, were placed. There were some ancient stocks in which were five holes for fastening feet, hands, and head.
In Ceylon, at the present day, an instrument similar to this is used, only the head is allowed to be free.