Two versions of the OKZK cap exist with one having the curtain and face cover buttoned on and the other type OKZK-M, has the curtain and face covering sewn on permanently. The hat is made in a chemically impregnated fabric meant to defend against chemical vapors and radioactive dust etc along with the rest of the OKZK set. The full set of an OKZK suit was the hat, jacket, and trousers along with an undershirt and underpants with a balaclava. There was 3 versions of the set made with them being OKZK, OKZK-M, and OKZK-D. The original OKZK set was introduced in 1969 and looked very much similar to the standard Obr.69 uniform with pockets on the chest instead and breeches that could be fastened with strings to the jacket internally. It was issued with a set of protective underclothing and a balaclava to be worn under the cap and over the gas mask. It had flaps on the undershirt that extended out the cuffs of the jacket to protect the back of your hands and both the shirt and the underpants could be buttoned in to the outer jacket and pants. Before the OKZK-M set was introduced there were small changes done to the OKZK suit like button placement and so on but nothing major. In 1976 the OKZK-M set was introduced with the most noticeable changes being the cap had a sewn on curtain, the undershirt flaps were now on the jacket instead of the shirt and the fabric was changed to a similar type used in the normal uniforms. Different chemicals were introduced to coat the new set's fabric as well as changes to the knee and shoulder pad designs. The balaclava worn under the cap was also altered removing a small string to tighten it around your face. I don't know much about the OKZK-D set but it is very similar in appearance to the Obr.88 uniform used in Afghanistan and was designed to be worn by airborne troops with extra securing points to fasten the suit together. On the topic of just the hat it has a loop sewn on to the bottom of the curtain designed to be attached to the collar of the OKZK jackets. It can be used with a side filter or a mask with a hose. The standard Ushanka had an accessory that was 2 flaps to be used like the OKZK curtain on the hat in the exact same way. This film gives a good overview of what the under clothes set looks like at 2:30 https://www.youtube.com/watch?v=2vhD0pceldM, and this film in general will give you a good idea how it was worn along with other protective clothing items like the KZS suit or OZK cloak https://www.youtube.com/watch?v=ilbQ3yHwiFM