Fatto generale: 1639 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1639 is the number of binary rooted trees with 16 vertices.
Trivia: 1639 is an unremarkable number.
Data storica: June 26th is the day in 1974 that the Universal Product Code is scanned for the first time to sell a package of Wrigley's chewing gum at the Marsh Supermarket in Troy, Ohio.