The gun had to be a complete pistol *in AW configuration, as defined FOR PISTOLS* on 9/13/94.  When it was originally assembled as a pistol, it must never have been a rifle, or sold as a rifle.

You can't make a pre-ban rifle into a pistol of any kind, and since "pre-ban" status requires the gun to have been complete, that means only complete pre-ban pistols qualify.  There weren't many of these, BTW.  Generally just the Olympic OA-93 and the Rocky Mountain Arms "Patriot Pistol".  The former uses a custom upper; the latter uses standard AR internals.

You can put any pistol upper on a pre-ban pistol.  There is no weight restriction (that applies to post-bans).

-Troy