I don't think !$ is properly understood as a readline feature. It's a sh/bash abbreviation. !! -> last whole command !!:1 -> first argument of last command !

I don't think !$ is properly understood as a readline feature. It's a sh/bash abbreviation. !! -> last whole command !!:1 -> first argument of last command !!:$ -> last argument of last command !$ -> abbreviation for above