Another WW1 gas mask I own, c. 1917. Combines elements of the British Small Box Respirator and German GM-15. Notable features include the exhale valve on the filter stem and the fact that it's basically a SBR face piece with a large mechanism plate. Carried in a metal can similar to the GM-15.
This is the second version of the mask, as shown by the support straps being on the inside of the face piece. Mine isn't in the best condition out of the ones I have seen, but it was owned by Chris Carey, and it's the one shown in the book "U.S. Chemical and Biological Defense Respirators: An Illustrated History".