I can't comment on why Jay's head shows an "X" but I'm very curious and would love to find something on Pontiac letterhead that explains it.
While writing my Firebird books, I had a chance to look at some original blueprints. According to the notes I took, the #48 blueprint has chamber volume of 66.16 cc while the #12 shows 69.27 cc. The chamber shapes of #48 and #12 were identical. The added volume comes from the fact that the #12 chamber is positioned approximately .030-inch deeper into the casting. Deck surface milling makes that a moot point!
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