Didn't many of the documents copied by photocopiers of that era start "yellowing" quickly? I worked for the National Archives in the 1980s and recall many documents looking yellowed.
Photocopying has gone through many generations. When I was in grammar school in the late 60's and still encountered them in libraries into the late 70's/early 80's, photocopiers produced negative images (ie a photocopy of black text on white paper) and actually used a photo chemical process inside (the sheets came out wet and you had to place them someplace to dry). Then came positive image photocopying on special paper (which would yellow over years), and finally plain-paper copying.
Here is a chronology of photocopy and printer technology
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.