David Bain supporter Joe Karam says a second set of fingerprints of Robin Bain's thumb support the theory presented last month that the sooty parallel marks were from gunpowder residue.
Police had previously refuted the evidence, with Assistant Commissioner Malcolm Burgess saying fingerprints from Robin Bain showed white patches on the thumb which suggested cuts from "handyman" work.
"Had these been powder marks or smudges as claimed, we would expect to see a complete fingerprint image," Mr Burgess said.
Mr Karam said yesterday what the police did not tell media was that there was a corresponding set of "elimination" prints, taken closer to the time Robin Bain died.
"You'll note I'm relying on prints that are on the police file."