As the clan chiefs, who made available the models for Kenneth Macleay's series of watercolours commisioned by Queen Victoria, generally supplied the costuming as well I don't think that we should read too much into the sardines-- sorry, salmon-- on the front of "Postie" John MacLachlan's jacket. If anything, I suspect that they may be worn as a "livery badge", indicating employment on the estates of the Chief of the MacLachlan.