The 6/12 orientation is easier to "see" than a 12/12 orientation is. In timing chain issues, it has usually been "normal" to use the 6/12 orientation when the engine is rotated before anything is replaced. So 6/12 it has always been presumed that #1 is firing then.
When my late machine shop operative mentioned that 6/12 is when #6 fires, which is 180 degree "out" from #1, I was skeptical, but at his age and level of experience, I didn't get past an initial question and his explanation.
At the time, he was talking about Chevrolet engines. I DID eventually find that all Chevy V-6 and V-8 engines are that way, in several Chevy factory service manuals. Plain illustrations and which cylinder was firing in each case.
NOW, if you consider the orientation in the middle 1950s when the B/RB engines were being designed AND the fact that everybody was looking at what everybody ELSE was doing back then, the similarities between the 1955 Chevy V-8 and the Chrysler B/RB engines become apparent. Just that Chrysler changed some things (usually, for the better) in the process. Like the taller deck height, front distributor, shaft-mounted rocker arms, as to the engine design itself. Cam duration for the original B/RB engines was similar to the engines, too. As was Ford's 352 cam timing.
Ford and Chrysler both had V-8s they could have worked off of, but chose to do something different and head in directions that were less-expensive to produce, would be performance competitive, so Chry apparently chose to follow the Chevy orientation, but make the engine capable of easy displacement increases and such.
Funny thing is that in some of the dirt track classes, builders were fusing 1" of metal onto the decks of Chevy 350s so they could run longer strokes and the long rods too. Which pretty much put them into Chry B/RB territory! This was in the 1980s. Smokey Yunick wanted the distributor up front, but never could convince Chevy to make that change. In later years, kits to use shaft-mounted rocker arms came out for SBCs, too. Lots of little things happened, back then, that made me smile.
Now, if the marks are at either orientation, in the B/RB engines, you're either correct or incorrect (180 degrees out). If incorrect, easy to change with the slot to deal with rather than having to clock the drive gear in. Which is a big plus!
Whatever works!
CBODY67
