This is small addition, it's two-seat Jastreb under designation NJ-21. It looks very similar to G-2 but it's J-21 with extra seat for instructor. NJ-21 was used mainly for gunnery practice.
In yugoslav army designation system N (nastavni) stand for trainer and J (jurišnik) stand for attack aircraft, so NJ is trainer-attack. Every aircraft, domestic or imported, received army designation (MiG-29/L-18, G-2/N-60) and some domestic built have only army designation from the beginning like J-21.
Some other designations:
L (lovac), fighter
NL (nastavni lovac), two seat version of fighter like MiG-21UM
T, transport
V (višenamenski), multipurpose (liaison, observation, light transport, basic training...)
I (izviđač), reconnaissance
IJ (izviđački jurišnik), reconnaissance version of attack airplane
HO (helikopter osnovni), basic helicopter
HN (helikopter naoružani), attack helicopter (naoružani=armed)
HT, transport helikopter