Mystery Nut--Anyone Identify?
 

Can anyone please identify the nut pictured at the following
site?

http://home.earthlink.net/~honda.lioness/id16.html

It's head is 12 mm across. It has some sort of locking
mechanism. It sheared off its stud, presumably due to age
and the vibrations that resulted while I was grinding off
the lower inboard control arm bolt on the driver's side.
Part of the stud appears in the photo. At the end of the
day, I found it beneath the cardboard on which I lie to do
this work.

I will be inspecting the engine compartment to see if I can
see a similar one.