The 104th Infantry Regiment was a regiment in the US Army, deployed to Boston prior to the war.
Echo Company of the 104th was tasked with search and seizure operations of all vehicles at the South Boston military checkpoint. Operations began August 1, 2077 and lasted until the Great War erupted in October that same year.
The appearance of skeletons dressed in military fatigues at the checkpoint indicate that at least some soldiers died there.
The 104th Infantry Regiment does not appear in the game, but is mentioned in Fallout 4.