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identify7T-Rex: are you sure about the dates which you cite? My recollection is that in the late '70s gold and silver went up substantially. This resulted in a melting of many coins, which were viewed as bullion in most grades.
The 21% interest rates did not occur until the early 80s. Then grading population reports showed which and how many coins had been melted. There was quite a rearrangement in the values of (even relatively) recent Franklin Half Dollars. These apparently suffered most from the melt.