My brother spent 25 years researching and documenting original owner Chevys from 1959 to 1974 with a goal of developing a comprehensive decoding service for Chevrolets. He launched the website about 1.5 years ago, I posted about it on this site. He went out of his way to state emphatically that his site (Classic Auto DNA) was a decoding service not an authentication service like PHS. He also put a lot of stuff on his site and reports warning about potential fraud, restamps, etc. His site even provided photos of original trim tags (from original owner verified cars) and stampings from various assembly plants to help people avoid "made up" cars. His serevice just never took off because a lot of his info is available if you buy all the right books.
During his site development we went to a large well-known auction and attempted to decode cars that were presented as "numbers matching" etc. We decoded (148) 1959-1974 musclecars - Impalas, Chevelles, Camaros, Corvettes. Out of those 148 cars presented as "numbers matching" we found 37 cars where the trim tag matched the current paint/trim/options on the car and had original stampings on the block. It was amazing how bad some of the block restamps were done. Once you know what to look for, they are not hard to spot.
For all those that commented on how there would be a lot of unhappy Chevy owners if there ever was a service to authenticate their car.....you are absolutely correct.
Fortunately or unfortunately, depending on where you sit, that will never happen.