Francisco- Be very careful, it may look good, but this seller has a reputation of selling bogus helmets, I almost got taken by him, and after a few months finally got my money back.
I do not think 450 MG Abteilung should have a helmet like this. Also the originals are crescent shaped according to the 3 photos I have seen. There is a good picture on pg 56 of the Wannenwetsch / Hilsenbeck Wurt book. Flasheart has a real neckflap you can see at
http://www.pickelhauben.net/articles/ersatzarticle.htm