(at around 1h 20 mins) Frank Long leaves payment for his room on the hotel registry. The top $1 bill is a modern "small size" Federal Reserve Note with a green seal. The first of such notes went into circulation in 1963, some 30 years after the movie's prohibition era, which ended in 1933. When small size US currency replaced "large size" currency in 1928, all $1 bills were silver certificates displaying dark blue seals and serial numbers.
After checking into his hotel room Frank Long tell the bellhop that he uses his Browning Automatic Rifle to hunt bootleggers, moonshiners, and anyone who "indulges in the illegal sale and drinking of liquor." BUT under Prohibition it was only illegal to manufacture, transport, or sell alcohol. It was NOT illegal to DRINK alcohol. And as a prohibition agent, Long would have known this law and fact.