>> M-n and M-p: were initially unused, became what they are now in Emacs 17
>
> M-n and M-p are still unused? Or did the convention
>
>     A major mode can also rebind the keys ‘M-n’, ‘M-p’ and ‘M-s’.  The bindings for ‘M-n’ and ‘M-p’ should normally be some kind of moving forward and backward, but this does not necessarily mean cursor motion.
>
>     (elisp) Major Mode Conventions
>
> not exist before Emacs 17?
>

No, M-n/M-p are of course not unused.  They were unused before Emacs 17 
(except in rmail mode).  The current convention for M-n/M-p started to 
exist with the binding of M-n/M-p in the minibuffer to move to the 
next/previous command in the command history in Emacs 17.  But of course 
at that point it was not formalized as a convention.