A gun cupboard, inscribed with William Larnach's name, was discovered at a Patearoa farm which was owned by a local family for more than a century.
The cabinet has a hand-written note inside, which reads: "Larnach's gun cupboard, bird's-eye maple wood".
"It looks very authentic. We're 99% sure it's authentic. It has the same catches that are used on other furniture at the castle," owner Don Hayes, of Fairfield, said on Friday.
"It would have been used to store long-barrelled musket guns, or black-powder guns. After Larnach died, everything was sold off ."
Mr Hayes is a pest controller with the Animal Health Board and bought the property in April as a Maniototo base for his possum-control operations.
"It was in a tool shed out the back of the property. I think it was just being used to keep tools in. Although, there were also a couple of gun cartridges in it, so it was probably used to store guns, too."
The farm had been the homestead of the Thurlow family since 1906, until John Thurlow died at the house, aged 77, in November last year.
Mr Hayes said he decided to give the cupboard to the castle after consulting Mr Thurlow's brother, Ron.
"I talked to Ron about it and he was rapt. He said John always knew it had some historical value."
Mr Thurlow (76), of Wellington, said his family owned the Patearoa property from 1906 until it was sold this year and that the gun cabinet had been there "just about as long as I can remember".
"I think the first time I saw it would have been in the late 1950s," he said.
"It was bought by a Patearoa farmer and gold miner, Bob Carr, at auction in the 1940s or early 1950s and, when Bob died in 1959, it ended up with my family sometime after that.
"It would be great if it went back to the castle, as far as my family is concerned."
Larnach Castle director Norcombe Barker was delighted with the gift.
"You don't get much of that, these days. It's dated 1898 and it's still in pretty good nick, considering what it's been through. We can't be 100% sure it's genuine, but it certainly looks like it."
The cupboard would be restored and installed in the castle gun room, under the front steps, Mr Barker said.
Larnach committed suicide with a pistol in Committee Room J at Parliament on October 12, 1898.
