KM-19 is one of the rather rare and hard to obtain Soviet oxygen masks produced after the WW2. It's design is very much like the one of US made A-8. A-8 were supplied in large quantaty to Soviet Union during WW2. It seems that Russians liked the design and tried to improve their KM-15 oxygen mask with features from the A-8.

KM-19, just like the KM-15, has a very basic design. Drawing on the experience with KM-15, KM-19 rubber face piece was made much larger - it covers not only nose and mouth areas, but also extends all the way under the chin. Overthehead strap was replaced with a much improved design - it is now made out of 2 straps (top and bottom). In the back, straps run through a wide rubberzied material strap which makes it more comfortable to wear the mask. In addition, top strap has a small extension strap made to run over the top of pilot's head. Overall, this new design allows a much more secure placement of the oxygen mask over the pilot's head. I would say that this is the best design Russians had before introduction of late KM-16 masks which made use of foreheand and chin connections to the leather helmet.

Another improvement over the KM-15 mask, was introduction of two side round exhale holes, just like those found on the US made A-8 masks. Holes are covered by rubber sponges which probably were placed there to prevent dust from getting isnide of the mask (I'm not a life support specialist so can't say if these sponges serve any other purpose).

Just like KM-15, KM-19 has a thin rubber oxygen hose. Mask is not equipped with any kind of pressure valve and thus is not suitable for high altitude flying. It also lacks any kind of inhale valve so there is a constant oxygen flow, which during inhale process mixes up with air located in the rubber bladder.

 

View of the KM-19 oxygen mask.
Close up on one of the exhale holes covered by rubber sponges.
On the front of the mask is a rather large stamp which says designation of the mask and its size. Also note placement of the exhale openings and thicknes of the rubber sponges.
Inside view of the mask. Note oxygen hole in the front of the mask and exhale openings.
Usually Russians painted metal oxygen hose connectors in black color, but on this mask it was left unpainted.
This picture shows the improved strap deisgn. Note that top and bottom straps run through rubberized material strap which makes it more comfortable to wear the mask. Also note the extension strap that runs over the pilot's head.
And this is how a pilot would have looked like when wearing KM-19 oxygen mask over the ShL-50 leather helmet...