I have a bolt which is in the joint of the bellows pedal assembly.
It's almost like any other bolt, but it has no threading, the head is
thinner (flatter), and there is a small notch near the top.

I went to a huge screw and bolt store in town which services many
industries in the state.  They have a huge warehouse of every type
of screw imaginable (I buy square-head nuts and bolts there).  When
I showed it to him the guy at the desk said he had never seen anything
like what I need

The original is long gone.  I haven't had it since I got the piano.
The previous owner had used a threaded, misfit bolt, but this didn't
work.  For one, the bolt kept breaking in half, and the other problem
was that the threading was cutting into the metal arm and expanding the
hole, causing the pedals to not fold back in the piano properly.

Damon Atchison
Damon66@juno.com