Chinese J-7 export to the US.

Probably used to study the nature of the J-7s since it mirrored so much like the Mig 21. They needed a full wing for Op For training.

 

The 4477th Test Evaluation Squardon was a specialized Aggressor training Squadron in the USAF but with some joint training and loaned members of the USN and USMC. They Operated actual "Threat Aircraft" as part of Red Flag Exercises.

@ Equasion They had a full wing.

Early evaluation of Mig 21 showed that it was a almost exact match for the F5 Freedom fighter which was then used as the main Aggressor fighter of the USAF/USN and USMC ( to this day)

The J7s were used along side actual Mig 21s as a simulated enemy fighter force. Some were paired off against American fighter pilots in there F4,F14,F15,F16 ecta ecta.... To get them up to speed on the advantages and detraction's of the Dreaded Mig others Served as spare parts and at least one ( not sure if J7 or Mig) was gutted and used as a photo op prop. They also operated Mig 23's. Along side the Chinese, Migs and J7s were scourced by the CIA from Arab defectors to Israel to nations who just no longer could deal with the demands of the Soviet Block.
Eventually before the official fall of the cold war Constant Peg was canned after a high ranking general in the USAF crashed a Mig23.

Rumors persist that although the 4477th is disbanded and declassified, Somewhere in the USAF is a squadron flying modern Russian and Chinese Birds for the same mission.
http://www.4477reaa.com/

 

During the 1980s the US sold China advanced equipment like the Hughes AN/TPQ-36 Firefinder Radar and a few J7s were probably not difficult to obtain.
China also sold J7s to countries in the Middle East and Africa. From there for the right amount of money anyone could get them.