I don't know about the letter not being cast right, there is definitely a difference between a letter O versus a U or a J, but I don't argue that there is little difference between them other than the letter. I think the casting number is more indicative of the design than the letter, but that's just my feel.
The 6 pack head wasn't cast any different than any of the other 915 heads, it was all in the machining. They had a different pushrod hole for the intakes so you could open up the width of the port was all.