again, thanks Andreas,
Having read to the end of `man gpart`, I encountered the useful -
gpart -vdg /boot/boot.bak
which I had lying around (good luck again). The output(s) confirm
that my manually fdisk'd partition matches the original.
So somehow I managed to corrupt hda4 at about the same time that
I blanked the MBR. I dont know how, but Ill accept that theyre
causally (not casually) related.
I looked a bit at gpart's partition-type-guessing code, it shouldnt
be too hard to work out which bytes are failing gpart's guesser.
Getting LVM to accept my hacks may be much harder.
So how big is an LVM super-block ?
I always have the (probably 1-way) fallback - a partion type change,
then an fsck to see what it finds.