The US Navy ND Mark IV Diaphragm Gas Mask was made in 1943-44 to replace the very similar mask, the Mark III.
The Mark III featured small oval-ish eye lenses, while the Mark IV features large semi-triangluar eye lenses, providing more field of vision.
The size of the Mark IV is "U" which means "Universal." This means that the mask can fit practically any size head.
One of the most interesting features of the Mark IV is without a doubt the filter resting behind the head harness, following the footsteps of the oddly designed Navy (Old Mk. II) Head Canister gas mask, which was designed in World War I.
The mask's facepiece is made of a thick and sturdy black rubber. Inside the facepiece does not feature an oral/nasal cup, but rather an "oral cone," which was designed so that the user's voice was more directed toward the voicemitter, providing clear voice projection.
The eye lens of the Mark IV are made of synthetic plastic, which is crimped onto the facepiece but a thick black semi-triangular eye lens ring. These lens resemble the lens used by the Army on some of their respirators made around the same time as the Mark IV.
The voicemitter features a thick metal guard with three sets of holes that appear to make an arrow-like shape.
The exhale valve that is attached right under the voicemitter is a MVI "spearhead" type exhale valve. The valve is attached to a curved metal tube by wire and covered with military grade tape. This certian type of exhale valve proved to be quite faulty, due to it being openly exposed. Often times, the exhale valves were missing on Mark I, Mark III and Mark IV masks alike.The Mark IV features an interesting MIII type head harness. Connected to the pad of the head harness is six metallic spring loops, which can stretch easily, making the mask perfect for a Universal size. Attached to the metal loops are cloth straps, which are not elastic, but can be adjusted. These straps, from my experience, are a bit of a pain to adjust.
Attached to the pad is a metal clip which looks like two rectangles. This clip so that the filter can be propped up, and not hanging below the user's neck.
The Mark IV filter is a MIX type filter, which is attached to the two hoses that run from the front of facepiece, right under the eye lenses, to the back of the head. This filter often times does not have the military grade tapes applied over the inlet holes. The filter also featured instructions on use and care.
The filter behind the user's head design was a blessing and a curse at the same time. Having the filter behind the head didn't limit how the user's shot a weapon, since the filter is not loaded on the left nor on the right, but having the extra weight behind the head seemed to be quite a burden.
The Mark IV's haversack is made of a grey colored duck material, which also has a shoulder strap. The haversack also featured two security flaps. One flap going horizontally, and one going vertically. Inside of the haversack is a loop for the anti-fog pruf paste canister.
The Mark IV also came with a cardboard (or metal) faceform, which also came with a set of instructions on putting the faceform inside the mask, and putting the mask in the haversack. The faceform has two thick ropes so that the user could tie a knot so the mask would not be loose when in the haversack.
Along with the faceform, there were also two semi-triangular cloths, which were meant to be kept on the eye lenses, to avoid damage, as the Navy was very strict on proper care of the Mark IV.
The Mark IV is a some-what common mask on the market, and can often be found for 30 to 60 dollars.
For more display of this mask see the following video:Edit