As this photo clearly shows, there is a big color difference between the plain edge coins from the Philadelphia Mint (upper coin) and the Denver Mint (lower coin.) You can see the copper core between the clad layers in the Philly specimen, but the Denver specimen shows an edge that is about the same color as the obverse and reverse.
The logical question, then, is, "if we can tell them apart, how come the grading services are slabbing them all as generics?" The answer is that the Denver coins actually display a minor secondary error of their own, in that the copper clad layer is supposed to show (according to experts I spoke with.) The theory is that the plating bath, which imparts the bright, shiny gold color to the coins, had too much dissolved copper in it, which was causing the solution to adhere to the copper core as well as the surfaces. Because this was considered to be an anomalous event that couldn't be relied on consistently, and because this plating bath could presumably be reproduced (faked) outside the Mint, the grading services elected to err on the side of conservatism and not indicate the mint of origin on the slab inserts based on the color of the edges alone.
The only sure way to prove which mint a plain edge Presidential Dollar came from is to find properly mintmarked coins from the same dies, and present them for grading and encapsulation in pairs.
The logical question, then, is, "if we can tell them apart, how come the grading services are slabbing them all as generics?" The answer is that the Denver coins actually display a minor secondary error of their own, in that the copper clad layer is supposed to show (according to experts I spoke with.) The theory is that the plating bath, which imparts the bright, shiny gold color to the coins, had too much dissolved copper in it, which was causing the solution to adhere to the copper core as well as the surfaces. Because this was considered to be an anomalous event that couldn't be relied on consistently, and because this plating bath could presumably be reproduced (faked) outside the Mint, the grading services elected to err on the side of conservatism and not indicate the mint of origin on the slab inserts based on the color of the edges alone.
The only sure way to prove which mint a plain edge Presidential Dollar came from is to find properly mintmarked coins from the same dies, and present them for grading and encapsulation in pairs.