One of the differences between the MiG-15 and the MiG-15bis, in addition to a newer and more powerful engine, is the gun configuration in the nose. It is not readily noticeable, but there are differences in the 23mm cannons and their fairings.
The MiG-15 was equipped with one N-37 37mm cannon and two NS-23 23mm cannons in a drop down tray on the bottom of the nose fairing. This was the standard configuration until improvements were made in the MiG-15bis. The MiG-15bis received two NR-23 cannons, which had a higher rate of fire and an improved ammunition system. The cowling also changed with bulged ejection chutes. The fairings on the newer guns were also shorter than their predecessors.