On top of the helmet is stamped P21 which I understand to be the heat lot code. There is some dirt in the air vent lugs.Has the original chin strap. On the inside right is several letters and 66.
It appears to be BG? The color paint makes it look like rust but there isn't any rust.
This is a very nice helmet and would be hard to upgrade. I will be listing some more helmets over the next few days. The item "KuK M17 Austria Hungarian helmet Isonzo Braun original WW1 WWI size 66 German st" is in sale since Monday, October 21, 2019. This item is in the category "Collectibles\Militaria\WW I (1914-18)\Original Period Items\Austria-Hungary". The seller is "bloodhound1999" and is located in Menominee, Michigan.
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