This is the air deflector for the US M40 gas mask that mounts the intake diaphragm valve and directs air up to an eyepiece to reduce fogging.  Valve not included.  It is difficult to install and isn’t likely to ever need replacing.  This must be removed and swapped to an opposite side when you want to move the filter canister to mount on the opposite side of the mask, or to use two filters at once.  This fits only medium and large US M40 gas masks.  One piece plastic, the photo with two is only for illustrative purposes to see both sides of the part since only one is needed in a properly functioning gas mask.