Pewter cup with Japanned tin box
The seller is playing up the rarity of the item, but I've see them often enough to question how rare they are. You can find the pewter cup without the japanned tin box even more often. Sometimes sellers will refer to them as "lead".
Also, note the size of the cup. Most CW cups were about 3 inches tall. In later years, the cups got smaller.
There is also currently a cup listed made of horn. While it is interesting and the materials were available, I know of no documentation for such a cup as PEC or NUG.