Fatto generale: 21103 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21103 is an unremarkable number.
Trivia: 21103 is a boring number.
Data storica: October 10th is the day in 1920 that the Carinthian Plebiscite determines that the larger part of Carinthia should remain part of Austria.