I really can't tell by looking at the pictures, they look like hyper's. The grain structure looks different from standard cast. They're obviously cast and they're 389 pistons from the position of the valve reliefs. They're not a cheap casting which is what I would expect if they had been switched. They also seem to have a polished surface which is what the hyper's have, looking like a forged. Where the break occurred is in the area where the rings would be most apt to dig in due to the thrust and slight rocking of the piston on the pin. The ring gap would not have been as critical with a standard casting and any qualified machinist would almost definitely have gotten it right. The problem is that guys have so much experience that they no longer read the instructions, they get arrogant and end up getting stung.
Maybe someone else can say for certain that the pistons are or are not hyper's by the pictures, they look like them to me. You could always take the pieces to another machinist and they could probably tell you.
Rich
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