It's German but when I first looked at the pictures, I noticed that it looks a bit like a real helmet in those pics. I couldn't resist and didn't show all the pictures. Anyway, here's the rest of them...
As jou can see it's a money-box and of course a lot smaller then a real helmet. Wish I also had the key...
This one's waiting for you Karel :
http://www.ebay.com/itm/Miniature-German-Trench-Helmet-M-16-18-Stahlhelm-Circa-1916-/120791596290" onclick="window.open(this.href);return false;