If you have a copy of the "Guide Book of United States Coins" commonly referred to as the Red Book go to the Index and look for "Proof Sets". If you scroll through the years beginning with 1968S and ending with 1990S, you will find a number of listings with "No S" listed for one of the coins. You see, all of the proof coins are supposed to have the mintmark for San Francisco (S), but in a few cases, a proof die was used that did not bear the mintmark. These coins are very rare and command a substantial premium. Unfortunately, there are plenty of imbeciles on BoobTube and SleazeBay who think that just because they find a circulated coin produced in the same year as those sets it is worth a bunch. They don't take the time to learn that circulated coins struck in Philadelphia did not bear the mintmark for Philadelphia (P). Chris
