Leg irons were used as a device to restrain the enslaved. This is a photo of a British merchant on the HMS Sphinx removing these restraints in the early 19th century. Britain had abolished slavery. British traders transported an estimated 3.4 million Africans to North America as slaves during the 245 years the transatlantic slave trade was active.
There aren’t many known photographs of slave ships. This is one of them, taken by Marc Ferrez in 1882.