I’ve always been curious, and am about to paint one - but what the hell are those canisters on the vet Kazak backpacks? I think Spetznas have them too...
One of the Spetsnaz has the cannister connected with a hose to the back of his... headgear. IMO it would make more sense if it was the front of his mask, but still: my guess is it is a breathing gas cannister (air tank). The size and color reminds me of composite / kevlar oxygen bottles one could see in an ambulance-grade aid kit (well, or the R-1 first aid bag, standard equipment of medically-trained emergency services, like the Fire Brigade, where I live). Does it make sense for me? From a realistic point of view, no. Veteran Kazak's tank is not connected to his mask. It is also placed very high, on the top of his backpack, and thus exposed to enemy fire. Neither Veteran Kazaks nor Spetsnaz have any BTS that this could've been representing. Have a look at the older Veteran Kazak sculpts. They had elements of clothing rolled and attached to the tops of their backpacks, one apparently having a quilted poncho liner / woobie, the other having something more plain-textured (a poncho / shelter half - maybe even a plastch-palatka, or simply a rolled-up blanket, if he's very old-school).
Guess you could think of things like oxygen for high altitude low opening parachute drops (BS Spetz), or for oxygen (or propellant) whilst using Flamethrowers (Vet Kaz), sure we could think of other things too :)
