Complex sentence: In 1964 Harrison introduced the electric twelve-string guitar into mainstream pop; his Rickenbacker 360/12 twelve-string was a prototype-only the second twelve-string guitar Rickenbacker ever made-was delivered specially to him during their first visit to New York City.
Split into simpler sentences: In 1964 Harrison introduced the electric twelve-string guitar into mainstream pop. His Rickenbacker 360/12 twelve-string was a prototype. Only the second twelve-string guitar Rickenbacker ever made, it was delivered specially to him during their first visit to New York City.