Armed police battalions (Schutzpolizei) during the Weimar Republic. Good for those who like their Pulp Adventures in the vein of Christopher Isherwood.
Too late for the 1919 Revolution, really you want Erhardts or captured Russian cars for that.
As the armed police battalions were designed to be a reserve military force (a way of hiding true military strength under the terms of Versailles) you could use these in a 'what if' versus Poland and or France in the 1920s. War with Poland came close and it was something the Reichswehr actively planned for.