While all SKS variants are, at heart, the same basic design by Sergei Simonov, there are a number of unique variants ranging from Russian to Chinese to East German to North Korean variants and beyond. Here, we will take a quick look at four of the more common variants in terms of production numbers. Do note that all four variants seen here are in chambered for 7.62x39mm, are semi-automatic, and fed from a ten round magazine. All four variants can also be found on the American civilian market.
First, we have the original SKS-45 as developed, built, and issued by the USSR. While originally developed for World War II, the SKS-45 arrived too late for use against the Germans, save for reports of a few early prototypes used for field testing. Instead, the SKS would find a relatively short life as a standard rifle for the Red Army after the war until production of the AK-47 could meet the demands of the Soviet military. However, the SKS-45 would continue to see service to some degree with the Soviets and later Russian Federation to some degree, including its current use as a ceremonial rifle.
Next, the Type 56 is a Chinese version of the classic Russian design. In 1956, the Soviet Union provided the People's Republic of China with a complete factory for production of the SKS carbine, including parts, partially assembled rifles, and Soviet factory workers to help train the new Chinese workers. Early versions of the Type 56 were identical to their Russian counterparts; however, some changes were made over the years. Most notable of these changes including changing the manner in which the barrel was fitted to the receiver and replacing the blade bayonet with a spike design (which was quicker and cheaper to produce and possible stronger). Large numbers of both the Russian and Chinese guns were sent to countries around the world as military aid during the Cold War. Furthermore, several versions of the Type 56 were produced for the civilian market, including shorter barrel length carbines and ones fitted with magazine wells for AK magazines.
Third, we see the Yugoslavian M59/66 SKS. The major unique feature of this version of the SKS is the fitting of a rifle grenade launcher and sight. By flipping the grenade sight up, the gas system of the rifle is shut off, disabling the semi-automatic function of the rifle and making it essentially a bolt-action. Then, a rifle grenade could be fitted to the launcher on the muzzle and a special blank round loaded into the gun which, when fired, would launch the grenade. Then the shooter could manually eject the spent casing and either load a fresh grenade and blank or flip the grenade sight/gas cutoff back down, chamber a live round, and use the rifle as a conventional firearm.
Lastly, we have the Albanian SKS, sometimes referred to as the "July 10th" rifle. This version is itself based on the Chinese Type 56 SKS due to the tooling for rifle being acquired from China, with which Albania had close ties to during 1960s and '70s. The Albania SKS features the Chinese-style spike bayonet as well as a unique set of furniture with a handguard which covers the entire gas system and a second trapdoor in the buttstock to allow storing an oil bottle and a cleaning kit. Additionally, the rifle features a charging handle similar to that of the Kalashnikov rifle rather than the knob design found on other SKS variants. Of the four common variants seen here, the Albanian is rarest of the group due to limited production numbers and large numbers of the rifle being destroyed as part of UN-led disarmament programs.