That's interesting, but what army did he serve in? If he was able to stamp photos as passed it would make sense to "pass" those he kept for his personal collection, for obvious reasons.
My guess is these fellows are "Imperials" and I suspect one of the battalions of the Black Watch.
If the cap badges are not obscured deliberately, then why can we not see a single glint from any of them, even though the angle of the balmorals on some of the men clearly indicate that we should? Blackened badges? I haven't seen any for WWI Scottish regiments myself.