I bought a 1957-D mint set card last year (there was no P card). The silver coins were the main reason I bought this mint set card, but I enjoy the cents and nickels for what they are... nothing spectacular, but decently toned. The images are by Bob Campbell, and I think he did an excellent job.
Before 1959 mint sets were packaged in cardboard holders. One card for each mint. Each card had holes to hold two coins of each denomination, one obv up one rev up. A paper cover wrapped around the card. It was glued to the card on one side and the other side was loose so you could lift it up to see the coins. This is what one looks like. Notice the paper cover at the top of the image.
Thanks Conder, I had not seen one before. What got my attention was the fact the OP showed two coins of each denomination. Seemed odd to have a mint set with two of the same coins in it. Now I know why.
Love those double mint sets. With unlimited funds, I would buy every single one I could find (as well as older proof/mint sets)