An diesem Tag vor 105 Jahren unterzeichneten Belgien und Luxemburg die belgisch-luxemburgische Wirtschaftsunion. Sie ist die älteste noch bestehende Wirtschaftsunion Europas und diente sowohl den Benelux-Ländern als auch der Europäischen Union als Vorbild.

That explains why used the same coins (other guy on the back)

The picture shows the Belgian and Luxembourgish delegates meeting in 1935 (source : Wikipedia).

Some trivia

* Luxembourgers voted in a referendum for closer ties with France but only negotiations with Belgium went through.
* Belgium and Luxembourg pegged their currencies as early as 1929. 
* Within the EU, Article 350 guarantees that the provisions of the EU Treaties shall not preclude the existence or completion of regional unions between Belgium and Luxembourg (as well as between the Benelux countries).
* Up until 1999, the two countries were treated as one entity in trade statistics.
* Towards the end of WWII, the union sought to cooperate with the Netherlands. In 1944, a customs union was signed, which would later become the Benelux Union.

That was in an era when we still had statesmen. These days, we have to make do with politicians.