The versioning of diags rootfs/kernels has always been pretty peculiar (and, by definition, harder to study, because we've never seen a diags update in the wild, nor have we ever heard of someone capturing one OTA, so we're left with what we've seen ourselves on a handful of devices) so I wouldn't obsess too much about it
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Good to know about your experience, though, it indeed makes a good part of the RP unnecessary on those devices
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EDIT: I don't recall the initrd's updater's error code OTOH for a downgrade ('attack', as the constant was lovingly labeled), but that's probably it.
And, yeah, I meant dd partition dumps/images
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