I figure this is a penny but I've never seen this before. It has a large "P" on one side with the word "uncirculated" above and the word "Philadelphia" on the bottom. On the other side it says "Treasury" at the top and "United States Mint" on the bottom. In the middle, there is a shield with a key at the bottom and a scale at the top. Anybody ever run across one of these?
Part of a 'mint set', which is a little different from a proof set. Check google or ebay for pictures of old 'US Mint Set's in the original packaging, and you'll see it. They also make a D for Denver.