I believe this is a trench whistle used by my maternal grandfather in the First World War. He was an NCO with the Canadian Expeditionary Force (43rd Cameron Highlanders) and would have been issued a whistle to relay orders during the noise of battle. He was also a Boy Scout leader but there is no Scout logo on the whistle which leads me to believe this was his trench whistle. I did a bit of a search and could not find a picture of a trench whistle with similar markings, so I'm doing some speculating here. I posted B&W versions but like these better.