The U.S. Navy Diaphragm Optical Mk I Gas Mask was based on the U.S. Army MI-I-5 for use with optical sighting devices.  The distance between the small circular eyepieces could be adjusted to accomodate different instruments and head sizes.  The filter clipped to the head harness behind the user's head, with two hoses drawing air up towards the eyepieces.  This example was made by Mine Safety Appliances in 1944.

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