First question -- what J-20 mockup at Changchun airshow? AFAIK we've never seen an actual J-20 mockup at any airshow. Are you talking about the FC-31 mockup at Changchun airshow instead? Or perhaps I missed something...
As for his claims....
I assume he is suggesting that the first J-XY/J-35 prototype is dubbed 31005.
But this actually raises a bigger question I've been thinking about.
What actually is the PLA designation of J-XY/J-35 going to be?
J-35 to me always seemed like it was a stand-in for the aircraft and was more a reference made by PLA watchers in relation to F-35 than what the serious name of the aircraft would end up being. If the aircraft is actually called J-35 I feel like it would be a bit on the nose.
More importantly, J-35 would be rather uncharacteristic of the PLA to skip its fighter designations from J-20 all the way to J-35. Of course, they have certainly skipped transport and bomber designations to reach Y-20 and H-20 respectively, but I feel like those were more understandable and reasonable because they were obviously seeking to have designation commonality/unity with J-20 and Z-20 etc.
But I can't see a reason why they'd skip all the way to "J-35".
The other possibility is that if the first J-XY prototype is called "31005," then could the actual eventual PLA designation of the aircraft end up being "J-31" instead? That of course would be very ironic as for many years there justifiably have not been any "J-31" but instead FC-31, yet would then be followed by there being a "J-31".
However at the same time going from J-20 to J-31 is quite a large skip as well.
That said, if the actual first J-XY prototype is called "31005" (assuming this guy isn't lying), then that is the only way in which the prototype serial number designation would make sense.
Personally, as far as continuity of designations go, I think that the name for J-XY might most "logically" be called "J-21" as the most likely name, or "J-25" as the second most likely name.
"J-21" because it follows naturally on from J-20. It seems like a no brainer for me, and the PLA has of course been happy to continue from J-10 to J-11 in the past.
"J-25" is a distant second, and would be chosen as J-XY would be a carrier based fighter and be ten ahead of "J-15". But as I said, this one seems less likely to me.
In other words, I wonder if the guy's claims is true or if he's assuming the first J-XY prototype is "31005".
If the actual PLA designation of J-XY is "J-21" instead, I would expect the first prototype to have a s/n of "21001".
Either way, I would just find it really remarkable if the PLA really skip all the way to J-3X designations for the J-XY, as I would have assumed they would be keeping the J-3X designations free for their 6th gen aircraft instead.
