Indeed, a nice set! I'd say it's a Feuerschutzpolizei buckle.
Thanks for your opinions
Erno, I think that the Feuerschutzpolizei buckles are black painted
I don't remember seeing a green finished Police buckle before. I have a green Postschutz and you will find green Bahnschutz so I guess there might have been green Police buckles.
I was thinking it was just discolored grey/black Feuerschutzpolizei paint, but maybe it is something different after all. Perhaps Ben or the others will know more about it.
Nice buckle Ivan, to be honest never seen a green one before mate, but of a mystery there! Nice buckle though