I went through some old programs and articles that I still have
and this is what I can recall. In 1977, Aretio threw one at 177
mph at Newport. On February 2, 1978, he threw it 180 at West Palm
Beach. At the same time, he also threw a smaller uncovered pelota
233 mph. Finally, he threw one 188 at Newport on August 3, 1979.
For the life of me, I can't remember which one I saw, the 174 or the 188.
Whichever one it was, the P.R. department had been plugging it for a couple of weeks; Aretio will try to set a world record, blah, blah, blah.
The event was held between games. To answer Joe V's question, there were no other players throwing, just Aretio. There was Chief Bailey behind him with a radar gun, a few people from management and some photographers. His first throw was reported at some speed that was short of the record. On his next throw, Chief Bailey looked at his radar gun then showed it to someone else who I can't remember and a few seconds later the announcer said it was a new world record.
Aretio was a big guy and could throw it really hard on the reverse. Since he wasn't in a real game, he could really fling it without worrying about where the ball was going. The ball was definitely going a lot faster than in a game, so it may have been legit.
But considering the amount of publicity the throw is STILL getting, it may have all been a setup. The only people who would know are the man with the radar gun and the person who may have told him to fake it.
Until someone comes forward with the truth, it may remain...an unsolved mystery.