Hello,
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.
They sell for a few dollars each.
Hope this helps,
Jason