Don, if you go to his web site and read the bio, you'll find a bit about the performance that let to the "King of high C's". When I found his bio this afternoon, had to read through the whole thing. Here is the part about the high C's:

"...the culminating event in 1966 was Pavarotti's Covent Garden appearance - with Joan Sutherland - in an opera that is legendary for its "nine high C's": La Fille du Regiment. For the first time a tenor sang out in full voice the nine high C's of Pour mon âme, quel destin!, which Donizetti had written to be sung in falsetto. The audience raved, the opera house was shaken by a sort of explosion that even struck all the members of the English royal family who were present at the performance. "