Many thanks to you all for your nice comments. Mecklenburg helmets are particularly attractive and certainly deserve attention on this forum. I think Philippe won't contradict me...
And thanks to Zebedeus for starting this thread.
Brian, for those who did not see the story of this M15 before, it was found in San Francisco. The seller - a layman - had two M15s: this Mecklenburg and a Bavarian infantry in poor condition. Unfortunately, removable spikes had been switched. I tried to explain to him but he did not want to believe me and I had to buy both helmets to get the complete Mecklenburg-Schwerin M15...