This may be a long shot.
I wonder if the MM stands for Messing-Müller in Koln-Ehrenfeld? If so it would be a retail tag I would think. Still in business, I found this store description on the web:
"Whereas initially mainly brass goods were stocked here, the range on offer has expanded continuously over the last 225 years: you can of course still get brass goods, but the main focus is now on selected home accessories from leading designers."'Messing' in German is 'brass' which would have been their main focus, not a person name. I also read that Messing Müller is one of the oldest family businesses in Koln. There is a web site but I couldn't get much out of it except that they are expending or relocating. http://messing-mueller.de/