This badge would have been worn continuously from 1896 until the introduction of Stabrite badges in the early 1960s. However, looking at the reverse and the brazed construction of the copper lugs with the all-important feet, I suspect that this is a 1914-era badge.

The Duke of Atholl was the only man in Britain allowed to raise a private army, so this might well be for his unit. However, other members of his clan/family would have been entitled to wear this.