There is some discrepancy in the published narrative for this family.
Malcolm McMartin b 1746 was in the 1st Battalion of the Kings Royal New York regiment.
John McMartin b 1755 was likely the brother of the above Malcolm, but the narrative is that he was brother to the above Malcolm's father - supposedly another Malcolm.
Child naming for the the two does not indicate a shared parent (mind you missing births/infancy deaths could be present).
I have seen the Letterfinlay reference before. It may be a forced fit in this case.
The elder Malcolm in Killin residing at Bowvachter married to Catherine Clark has a son John b 1749, Malcolm born 1756, and a Peter b 1752 which could fit the family.
There is another elder Malcolm in Killin residing at Tirai married to Elizabeth McMartin, but only has female children with one unnamed birth, so not a likely candidate.