Detlef,
To me this looks like what is often referred to as a "Chain of Office" or a "Livery Collar," such as worn by government official or a Mayor of a city. There would be a medallion of some sort hanging from the central "key." These chains are quite long, hanging around the neck, resting on the shoulders and across the chest.
If you google "Chain of Office" you will see some very lavish examples in gold, etc. However, your example might be for a city councilor or a mayor is a small burg.
Cheers,
Ian
P.S. Are you sure it's brass and not gold?