I have observed the same thing too about the metal collar around the nozzles. Its thicker on the early planes but I thought that it was because of an earlier version of the AL-31FN.
You know, I'm not sure if the 100 engines recieved are vanilla AL-31FN, AL-31FNM1 with TVC, and here is another very likely possibility, AL-31FNM1 without the TVC (the TVC nozzle failed longevity tests with only 750 hours instead of the 1000 hours the PLAAF specified).
60185 and 60187 (probably with 60186 in between) are NOT early 100X protos repainted. They have the three door front gear configuration seen in later models. The first three door jets appeared with the FTTC (the J-10 with the PL-12 pic with service trucks nearby), but that was after the initial batch of two door jets were assigned to it (late prototypes, preproduction models and early production model).
I do believe that this unmarked plane here, with the metal nose for flight telemetric equipment, is the first AL-31FN powered J-10.
I need to add that based on photos, 1006, 1008 and 1009, plus two seater 1021 and 1022, have the thicker metal collar around the engines. I think there is more than just five WS-10/WS-10A powered prototypes out there.
- but for me it's a standard AL-31FN and from prototype 1003 to the later serial machines this "golden" or metal part in front of the nozzle has been redesigned.
