"What is a legal PreBan AR Pistol?"
>>A Pre-Ban pistol is one that has been built into a pistol prior to 9/13/03. The burden is upon you to prove this should the need arise. The restrictions of the ban do not apply.
"But how would a person legally own or purchase a PREBAN AR pistol??????"
>>As long as your state allows pre-bans, mine does not, you may legally own it. Just purchase one from a seller. Expect to pay $500-$1000 more than a pre-ban rifle.
"If the original owner/builder was to "register" it, then wouldn't any AR being built have to also "register" it???????????"
>>There is no registration of the pistol that you refer. That applies to Title II firearms.
As I understand it, a pistol, whether pre or post ban, can be made into a rifle (butt stock and >16" barrel) then back to a pistol at anytime. A rifle can not be made into a pistol, ever. You can, however, take off the butt stock, but the barrel must be >16" at all times.