Though a prototype for what would have been a new model , it never entered production .
A testbed aircraft -- a stock MiG - 21SM refitted with the uprated R13F2 - 300 turbojet .
Though a prototype for what would have been a new model , MiG - 21SM never entered production .
A testbed aircraft -- a stock MiG - 21SM refitted with the uprated R13F2 - 300 turbojet .