I'm going to be interested in the correct answer, but I am going to have an intuitive guess and say it's naval or AA...I don't think they were using alot of that calibre in the trenches then...but like I say wild-ish guess...
The Dawn Patrol was presented last weekend at the National Museum of the USAF. It had fly-bys of replica WW I fighters, dog fights, and such. There were a lot of re-enactors dressed in WW I costume as well and many vendors. I didn't get to the venue until late in the afternoon and as a result, missed most of the fighters. I managed to take a picture of one, but I'm clueless as to what model it was. I think its a Nieuport 17 as the squadron logo was for the Lafayette Escadrille used after April; 1917 (Sioux warrior) and the Escadrille was known to use this type of fighter.The roundels on the wings though were not French as one would suppose but American. Maybe that was done on purpose to reflect that once America joined the war, the Lafayette pilots joined the American Air Service. Pity there was no minder around whom I could ask.
I would not buy one nowadays. Considering how many masks may have been made they crop up pretty much weekly on eBay for £500 or so and I guess gullible Americans buy them thinking they are genuine.
Usually from sellers that have a lot of bad feedback and upon googling are found out to be traders of fake antiques etc.
Pisses one off.