I may be wrong, but I think that is for the optional (later) key in the lock switch for a something or other?
They all seem to be "universal" now.
The one I put in had a solid finger pick up for the horn ring instead of a nice roller ball like the originals do.
Cheap.
All I could find for the later A bodies were like that.
However, I recently bought a earlier B body one and it had the correct roller ball.