The brothers enlisted in the Canadian Army and appear to have served in the 10th Battalion Canadian Expeditionary Force by the badges on their uniforms. The 10th Battalion was formed from the 103rd Regiment (Calgary Rifles) which was later known as The Calgary Regiment.
When Colonel Ludlow begins writing "Am happy" on the small chalk board, the letters "Am" are initially very large and straight. In the next shot, they are much smaller and angled upward.
Susannah: Tristan, I have nowhere to send this letter and no reason to believe you wish to receive it. I write it only for myself. And so I will hide it away along with all the things left unsaid and undone between us.