The two 17-year-olds, killed in 1977, were seen leaving a pub with two men on the night they went missing, a court has heard.
Retired police constable John Rafferty, 57, was on duty outside the pub on the Royal Mile on the night they were last seen.
He said the victims and two men walked away together.
The court was told that the witness had been shown a selection of photographs by police in October 2004.
Mr Rafferty picked out a photograph labelled K which was agreed to be Sinclair.
Asked how sure he was that the person in the photo was the man he saw outside the pub, he said: “I couldn’t be 100 per cent sure”, but he added that he was “60 per cent sure”.
He was giving evidence on the fourth day of the trial of Angus Sinclair, 69, who denies the charges.
The trial continues.