It was a tossup whether to put this here or in the rarities post, but since it's not hens' teeth rare, just thin on the ground, it wound up here. Until 1866, when it was annexed by Prussia for staying neutral in the war against Austria, the city of Frankfurt am Main was a free city with its own standing army. If a fellow managed to stick it out for 15 years he got one of these.
Established by the City Council in December 1840, it was awarded for " faithful and reproach-free performance in the line military of the Stadt." The dies were cut by Tomschütz of Frankfurt and the medal was produced by the Herzogliche Nassauische Medailleur Zollmann in Wiesbaden.
Considering the population of the city was around 55,000 at the time there wouldn't have been much call for them.