They were indeed real, the Ringvereine (ring clubs). The first ones were initially founded in the 1890s (by former prisoners trying to help each other and their families) for example the “Reichsverein ehemaliger Strafgefangener” and the clubs organised under an umbrella organisation in Berlin (Ring Berlin). Finding work or a place to live was hard and this gave them a possibility to network; the rules of the clubs demanded a life away from crime and were harshly enforced.

In the beginning of the 20th century, especially after the first world war, they shifted more to something we nowadays would likely call organised crime (but of course holding up the facade), smuggling, extortion, prostitution, theft.

One of the clubs was Immertreu (always loyal) led by Adolf Leib, known as Muskel-Adolf (Muscle Adolf), coming to a spotlight in the Immertreu-Process of 1929; Immertreu was forbidden but it's members got only small relativly mild sentences because the members practically didn't witness against each other. A purported annecdote is that someone stole the lawyers coat during the process and it was delivered back to the owner later with greetings from Immertreu. Muskel-Adolf was an advisor to Fritz Lang for M.