@Robby:
Do they have a blank page in there? I don't actually have any japanese jumps to look at, so I wouldn't know. That's news to me, but that makes sense. I'd always just assumed there'd be an ad or something on the flip.
Otherwise I'd assume that since the chapters read right to left, rather than left to right, then the first page being a splash or a spread doesn't affect anything at all, the magazine just accounts for it. How it works in the trade volumes at least. (And why there are SBS pages in the first place, to accommodate for pacing gaps.)
Slotting in a page 0, basically. (Or if it helps, the story usually starts on page 2, not page 1.) But if there's usually a blank page then I dunno. SHould be easy for Oda to account for in the Name phase of things unless something went really screwy.
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A blank page is how it comes up in the translated digital releases and most of the raws that come out, but yeah, there'd be ads on it in the magazine release. The ones opposite normal covers get SBS and fanart pages in the volumes, while the ones after colour spreads generally stay mostly blank in the volumes, with just a little One Piece logo or sometimes a Pandaman highlight in the bottom right. I say "blank page" because the stuff on it (at least before the volume release) is disposable and replaceable and doesn't really ever make it out of Japan anyway.
You're right it would definitely depend at what stage of the storyboard/art process Oda was on when this came up if it was any trouble to change or (if unplanned) will have any noticeable effect on the page layouts like the chapter 829 outlier. I'm also aware that this doesn't really matter at all and is about the most idle speculation possible, but hey, what else is there to do for two weeks.