The SG43 was a medium machine gun developed in Russia during World War II (1943, as you might have gathered from the designation). The initial development was done by Degtyarev, who was responsible for the current light (DP28) and heavy (DShK) machine guns in Russian service. His designs used a pair of locking flaps to operate, and the medium DS39 developed in 1939 was no exception. The gun was not successful, though, and production ceased with only a few thousand made. The design was modified by Goryunov into one where the entire bolt locked into a recess in one side of the receiver.
The result was the SG43, which served well through the later years of WWII, and remained in frontline Soviet service after the war (unlike most designs used in the Great Patriotic War). The gun was eventually updated as the SGM, with the major changes being a redesign of the headspacing and barrel locking mechanism and a change in location of the charging handle. The most distinctive change was the adoption of a slightly lighter fluted barrel for the SGM.
We have a series of photographs of an SGMB (the vehicle mounted variant) disassembled by Aberdeen Proving Grounds in 1965. They do a good job showing all the small internal parts, and should be a good reference for anyone with parts kit for a gun in this family.
An SGMB disassembled by Aberdeen Proving Grounds in 1965. Also available in high resolution for download: SGM Aberdeen photos (hi-res)
This SGM was discovered in a cache of IEDs in Afghanistan by a reader serving in-country. He was kind enough to send these photos: SGM found in Afghanistan (hi-res)