I know little about the science of soaking, but isn't an alternative explanation possible? Maybe this really was a P350 back. The glue used to adhere it to an album caused the paper to deteriorate to the point that the multiple layers of the card's cardboard separated once soaked. It does not leap off the page to me as an obvious sign of duplicity. Seems like a lot of effort to conceal a re-backed card, which wouldn't really be necessary as most collectors wouldn't even notice.
I don't doubt such chicanery goes on, but there seem to be possible multiple explanations here. Unless you found the original back also underneath the re-back.
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