Per (PeteCresswell):
A swivel snap hook.

Now *that* was quick.

But it doesn't seem tb working Google-wise.

I get hundreds of hits on 'Swivel Snap Hook' but they're all a
single hook with some sort of bail to keep it from coming
un-hooked - whereas the essential feature of the one I'm looking
for is that there are two hooks that overlap when the thing is
attached to something.

viz:
http://www.flickr.com/photos/48262653@N00/488669232/


Likewise "Snap Shackle" returns many, many hits - but none
relevant. The only superficially-similar one is a
trigger-release snap shackle which is sort of the opposite of
what I'm looking for in that it is designed to release quickly
and easily... whereas mine is inherently/intentionally difficult
to release

Only one I found so far is by a Chinese manufacturer who seems to
lump them in with "Snap Hooks": http://tinyurl.com/298jly as in
the large one on the upper right.
--
PeteCresswell