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The investigation instigated by Dunedin Mayor Dave Cull revealed the shackles were leg irons and could have been used on people or horses and came from a cave in Portobello Rd, near the sea scouts' boat shed.
Dunedin man Steve McCormack, who sold the shackles to the council, said he and his late brother removed them from the cave in the 1970s.
The report released to the Otago Daily Times yesterday said the irons appeared to be from the late 19th century, the same period when Maori prisoners were held in Dunedin between 1869 and 1881.
''We may never know exactly how these leg irons were used, but they are associated with that period of history in Dunedin when construction work was happening in the Anderson's Bay area using Maori prisoner labour,'' the report said.
Several horse hobbles and leg irons from the period were researched for the investigation and Mr McCormack and local historians were interviewed.
Horse hobbles were often mistaken for leg irons and while the shackles seemed ''too big'' for human legs, 19th-century leg irons varied greatly in size and design, the report said.
''The conclusion is that these items are leg irons, and could have been used on either people or horses.''
Museum curator Sean Brosnahan said he thought the cave had been cut into the rock and used for storage, or shelter, in the course of the development of the road.
Dr Matt Schmidt, of the Historic Places Trust, said the caves were used by dairy operators to store horses.
Historian Peter Entwisle believed the cave was used for storing explosives during the causeway's construction.
Bill Dacker, named in the report as the leading authority on the Maori prisoners' experiences in Dunedin, believed the caves had been large enough to house horses.
Mr Cull pulled the leg irons from an auction and bought them last month with money from the mayoral fund.
Mr Cull had been talking to runanga in Taranaki and the irons were ''hugely symbolic'' if they restrained prisoners or horses.