During Nucky's executive meeting about the end of Prohibition that takes place in 1931, there is a reference to the building of Rockefeller Center. The 22 acre Manhattan landmark was built over a period of nine years (1930-1939.)
Some of the photographs of young girls on the Commodore's desk were taken in the 1860s and 1870s by Charles Dodgson aka Lewis Carroll.
Willie Thompson mentions that he studied at St. John's Law. Terence Winter who created the show also studied at St. John's Law and actually practiced law for a few years before making the jump to television.