Complex sentence: Piper built a prototype PA32-260 with IO-360 engines on the wings, a tri-motor which was the proof-of-concept aircraft for the twin engined, retractable gear version of the Cherokee 6 which is called the Piper PA-34 Seneca.
Split into simpler sentences: Piper built a prototype PA32-260 with IO-360 engines mounted on the wings. The tri-motor aircraft was the proof-of-concept aircraft for the twin engined, retractable gear version of the Cherokee 6, the Piper PA-34 Seneca.