Regarding the "M", and somtimes seen "MM" stamps, I'm going to guess that it may have been for Max Metzner of Zella Saint Blasii while post WWI could be for Max Moller, a barrel maker of Zella-Mehlis. For now I'm not sure when Moller entered the scene but I can't locate him in the 1907 Zella Saint Blasii/Mehlis phone directory. He many not have had a phone.

Kind Regards,

Raimey
rse