Well done - that's it! Ben Stack has a very distinctive outline, but nearly all photographs are taken from the north or north-west - which might make you think that I had reversed it, again.
But I found this one below, which is labelled "Ben Stack from Meallan Liath Beag", which is about 17km away SE of Ben Stack.
It's clear that my Dad was on almost the same bearing from Ben Stack. From the map, I realised that he was much closer, on the high ground to the SE of Loch na Mucnaich at the bottom right of the map below. The 719m peak of Ben Stack is at the top left.
On the shore of Lock Stack beside Airdachuilinn, above the centre of the map, is the house where my Dad's uncle and later his cousin stayed when they were head gamekeepers on the estate there. It's an area that he loved.