If there were a thirty year old original it probably would have an obvious aged look to it. There is no thirty year old original because these documents are obvious fakes and therefore CBS will do whatever is necessary to avoid producing the original which could easily be shown to be fake.
Best I recall, Rather never claimed to have the original. I believe CBS is also going with a copy...which should have set off alarm bells in their "expert"s brain the minute they showed them to him.