There has produced and busily test flied four experimental prototypes of J-20 stealth fighter jet nos. 2001, 2002, 2003 and 2004 and six prototypes nos. 2011, 2012, 2013, 2015, 2016 and 2017. There has been news that trial series production of J-20 will begin in 2016 and trial service will begin in 2017.
The serial numbers of prototypes experimental or not are 20xx, but that of the brand new is 2101 to show it is another type of J-20 made in series production. They will all bear the numbers of 21xx instead of the numbers of 20xx for prototypes.
“Vital Breakthrough in Development of J-20: Emergence of a brand new series-produced J-20”