The gun was handed in at Tonypandy police station in South Wales on 22 September 1995 - five months after Jonathan Jones, 37, was jailed for life for the murders.
Police traced the single-barrelled pump action weapon back to a farmworker, Jeff Ayres. He was known to Harry Tooze, 64, and his 67-year-old wife Megan, who were both shot in the back of the head at their farmhouse in Llanharry, Mid Glamorgan, in July 1993.
Mr John Rees QC told the three appeal judges that police investigating the murders questioned Mr Ayres, who lived near the Toozes, shortly after the killings and he denied owning a shotgun.
"This was a lie. At the time of the killings it was hidden in his attic and could have been used in the murders," said Mr Rees, who added that Ayres had worked for the Toozes shortly before the murders.
Mr Rees said the gun, which had been offered for sale with ammunition for pounds 30, was handed in to police by a colleague of Mr Ayres. He wanted nothing to do with it because it had allegedly been used in the killings.
The disclosure came at the end of the third and final day of submissions by Mr Rees to Lord Justice Rose, Mr Justice Dyson and Mr Justice Gage.
Earlier in the day, he had told them that Jones did have an alibi, but it was treated unfairly by the judge at his trial.
Jones gave evidence at Newport Crown Court that on the day of the murders he was at his flat in Hollingbourne Towers, Orpington, Kent, and spoke to a lift engineer there.
From a series of photographs shown to him by police, he picked out a man working on the lifts at the flats that day who was not a regular member of the maintenance team, said Mr Rees.
Jones said he remembered speaking to the man, Ronald Bell, on Monday, 26 July, 1993. Mr Bell told him that the lift would be back in operation by the following weekend.
Mr Rees said Jones judged the time he spoke to the engineer as between 1.00pm and 1.30pm because when he reached his flat either Neighbours or Home and Away, the Australian dramas, was being shown on television.
The prosecution had alleged that Mr Bell could not remember the conversation and all three lift engineers believed they would have reached the flats later than 1.30pm, after their lunch break.
Mr Rees said: "What an incredible piece of luck this was if this was a false alibi that he should have picked out Mr Bell rather than other men he had seen working on the lifts for some considerable time, but who were not there that day."
He said the trial judge, Mr Justice Rougier, had directed the jury on the prosecution argument that it was unfortunate for Jones that the arrival time at his flat did not coincide with the workmen's account and that this was the one day they were not there because of a late lunch.
"The way he dealt with the timing was unfair on the evidence and circumstances which emerged. In any view, the timings were close," argued Mr Rees. The difference was between 1.30pm - the time given by Jonathan Jones - and 1.45pm, when the lift engineers said they returned.
Mr Rees said the judge also placed insufficient emphasis on the "unique features" of the day related to the police by Jones, which he could not have known about unless he had been at the flats in Kent.
The hearing was adjourned until today.Reuse content