The question we want to answer about this sterilization is why? The break up of Yugoslavia and ensuing conflict, very similar to today’s conflict in Syria saw an enormous diversity in the trade and manufacture of small arms, going in and going out of the conflict zones as all the various armed groups tried to get ahold of weapons to arm themselves with. Sterilization almost entirely is done to allow plausible deniability to the presence of a country’s small arms in another location. China in particular made entire batches of small arms with dedicated markings such as M21 on SKS’s, complete with actual serial numbers. These were fed into Cambodia, Vietnam, and Laos during the Vietnam War. This is a photograph I took of while in Burma of one such example-
Once the manufacturing difficulties of non milled receivers had been overcome, a redesigned version designated the AKM (M for "modernized" or "upgraded"; in Russian: Автомат Калашникова Модернизированный [Avtomat Kalashnikova Modernizirovanniy]) was introduced in 1959.  This new model used a stamped sheet metal receiver and featured a slanted muzzle brake on the end of the barrel to compensate for muzzle rise under recoil. In addition, a hammer retarder was added to prevent the weapon from firing out of battery (without the bolt being fully closed), during rapid or fully automatic fire.  This is also sometimes referred to as a "cyclic rate reducer", or simply "rate reducer", as it also has the effect of reducing the number of rounds fired per minute during fully automatic fire. It was also roughly one-third lighter than the previous model.