Wonder if it contains Asbestos, like the WW2 British ones did, soldiers had Crocidolite in theirs whilst Civilian ones had Chrysotile. Oh and the snow in the film was Chrysotile so happy fcuking Christmas Bing.
Bit of an old thread, but what you have is the facepiece of the American Small Box Respirator (ASBR), it was based on the British Version, however unlike the British version it suffered many problems which resulted in them only being issued for training purposes only which makes them quite rare today. As has already been said, it won't be worth much as its incomplete however you should be able to get at least £20 out of it.
I seen a British facepiece for sale last week for £20... and the British ones do seem to be more sought after and more expensive. Typically re-production equipment like is quite expensive due to the time and money used to make it, genuine American Small Box Respirators can be purchased on the US ebay relatively cheaply, although they prices are increasing with the 100th anniversary of WW1 approaching.