This SKS is a Russian semi-automatic carbine, designed in 1945 by Sergei Gavrilovich Simonov. It is formally known as the Samozaryadniy Karabin sistemi Simonova(Self-loading Carbine, Simonov's system 1945), or SKS 45. It was originally planned to serve as the new standard issue weapon for the Soviet military forces, alongside Mikhail Kalashnikov's new AK-47 design and was set to replace the Mosin-Nagant series of bolt-action rifles and carbines that had been in service since 1891 and chambered for the expensive 7.62 x 54 mm round.
As mass production of AK-47 rifles increased, the SKS carbine was soon phased out of first line service. The carbine was quickly replaced entirely by the AK-47, but remained in second-line service for decades afterwards, and remains a ceremonial arm even today. The SKS was widely exported and produced by the former Eastern Bloc nations, as well as China, where it was designated the "Type 56" (and, in modified form, the "Type 68"), East Germany as the "Karabiner S" and in North Korea as the "Type 63". It is popular today on the civilian surplus market in many countries. The SKS is chambered for the 7.62 x 39 mm round, an intermediate cartridge which went on to be used in the Kalashnikov-series weapon.
From the photograph on the right we can tell that this SKS was manufactured in 1951 at the Tula arsenal. If one looks closely, under the Tula arsenal marking(Star) a box with a diagonal line through it can be seen. This indicates that this SKS has been arsenal refurbished some time in its history.