Stocks are a fastening device similar to the modern equivalent of hand cuffs, but are fixed in position. Leg stocks would go around the prisoner's ankles such that they can not remove their feet and thus can not move from their position. Additionally, stocks generally didn't allow for any flexibility of movement because of how they were designed.
Leg Irons is a similar concept that is more similar to our modern hand cuffs in that it would clasp around the ankle and either bind the prisoner to a weight or a solid object (such as bars or the wall of the cell.) The advantage for the prisoner was that it allowed for leg movement, but was also more difficult to construct (since leg stocks could simply be made from wood, but leg irons required a more complicated metal locking mechanism.
Most likely it was the wooden variety and was simply translated later as leg irons to express the meaning more clearly. For a period of time, stocks were more commonly used as a public punishment to keep someone hunched over and in place with their head and hands fixed and unable to protect themselves from passers by. The use of leg irons may have been to differentiate from this more recent use of stocks.