Love that pic. My dad was a gunsmith in Joliet during that time. He had a Chicago policeman bring him 4 Thompsons the next day to service. Same Chicago policeman picked them up and paid him $5 cash for each gun. That was a lot of money in those days.
Dad said he knew where the guns (and money) came from but that he never had anybody but a policeman bring the guns in for cleaning and servicing.