Wow... That's some Sherlock Holmes stuff ☺How do you come to 71??? It's in fact aircraft number 21 and no. 21 is simply the individual aircraft number within that unit. In this case we know that it is the 66th Brigade at Dingxin (even if by the old standard pattern it would be the 176th).
Serial system: 78271 = 78x7x denotes unit ... seems as if here the pattern is 787-721 = 66 aka 66th Brigade and xx = the individual aircraft number.
We know that this unit is a mixed blue force unit flying J-16s, most likely J-10C and also J-20 ... and all serials we know so far show that the J-16 have 78071 to 78078 aka xx = 01-08, the next block of numbers is missing (so maybe J-10C) and 78271-78276 are the known J-20, so 78271 is in fact the very first J-20 assigned to this unit.
Deino
Thanks for the insight.