They look like a perfectly round cent, but they have NO features on EITHER side. Becasue they were never struck. They are ready to be struck, but not struck as of yet.
I have only found one, so I kept it. I think it is cool looking.
Found one years ago as a kid. Reminded me of the knock outs you get from metal electric boxes but its copper colored. I kept mine but dont know if there is much value to them since there is no identifying marks on them.
In the process of preparation for coinage, the blanks are first punched from a strip of metal and then milled to upset the rim. In most instances, first process pieces(blanks without the rim) are slightly more valuable than the finished planchets(blanks with rims). If is a penny planchet, with a good digital jewler scale you can weigh it to see if it is copper or a zinc planchet.
a copper planchet will weigh 3.11 grams
a zinc planchet will weigh 2.5 grams
I have found 5 blank planchet out of 2.2 million pennies searched, so roughly the odds are 1 in 500,000 pennies. Out of the five, four of them were zinc with one being copper.
The picture below is a zinc blank plachet I found.
Over the years, I've found quite a few of these, mostly cent planchets. I'm close to having a roll of them. I also have a dime, quarter, and half dollar planchet.