[decooney]

The 428 in my previous car had that, and a friend's 390 X block had that. I researched it through all the best FE books and back through different Ford people. We actually found several blocks like it thereafter. Most said the same thing that many of the blocks came from the same casting as regular 427s or something to that effect, including markings, but were ultimately machined as 390s or 428s - if that makes sense. It was kind of a maze of responses and the more people we asked the more it seemed like some things they did back then were just kind of intermixed to just get things done however it was needed at the time.