no its not, ak 74 fire the 5,45 x39 probabyl developt just before 1974

look at the drawing , its much smaller and available today anywhere - this would be no problem
http://www.municion.org/5_45x39/5_45x39.htm

the 5,6x39 is a 7,62x39 kalaschnikov necked down to 5,6 mm in the early 1960s
http://www.municion.org/5_6Vostok/5_6Vostok.htm

the cartridge was loaded once by sako and lapua as the 220 Russian but now you find only lapua brass

the tula ammo factory made it with steel cases http://tulammo.ru/en/production/vintovochnye-patrony/patron-5-6-39/

3,5 gramm bullet at 900m/sec

here for comparison 5,45x39
http://tulammo.ru/en/production/vintovochnye-patrony/patron-5-45-39/

and 223rem
http://tulammo.ru/en/production/vintovochnye-patrony/patron-5-56kh45-tyazhyelaya-pulya-/

problem is nobody import this cartridge anymore from russia