Joe tips the train porter with Chinese square-holed cash coins. That style of coin hadn't been minted for 25 years in 1936. In fact, one of the coins appears to have been issued by Emperor Sheng Zu (1662-1722), based on the tell-tale four dots near the rim. Of course, the joke of him leaving a strange tip wouldn't work as well with less-recognizable contemporary Chinese coins, which didn't have the square hole.