Hecho general: 81604 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 81604 is a boring number.
Trivia: 81604 is an uninteresting number.
Fecha histórica: June 4th is the day in 1896 that Henry Ford completes the Ford Quadricycle, his first gasoline-powered automobile, and gives it a successful test run.