Complex sentence: The twin-tails originally fitted were also later replaced by a large single tail fin assisted by smaller auxiliary fins and with their new engines and tails, they were redesignated HD.321.01 and HD.321.02.
Split into simpler sentences: The twin-tails originally fitted were also later replaced by a large single tail fin assisted by smaller auxiliary fins. With their new engines and tails, they were redesignated HD.321.01 and HD.321.02.