My grandfather was a messenger/chauffeur/ambulance driver during WWI, an Irishman already skilled w/ machines(including cameras) serving in the British Army. He served mostly behind the lines, but his luck ran out when he got a bad dose of mustard gas. Silver lining is that at the hospital in England he met grandma who was a nurse, which eventually resulted in me being me!
He told me about the American Indian motorcycle he loved to ride. But it looks like he rode some other brand as well that had a belt drive. And a third bike also w/ a sidecar for his chauffeur gig which I think is different than the other two. They ALL look more like big bicycles w/ a motor! Such dainty forks and wheels AND bicycle horns.