Why are there so many more un marked pennys and d pennys in circulation than s pennys?

The pennies marked S are minted in San Francisco. These are proof pennies only. There are very few in circulation. The only way they are in circulation is if some one breaks a proof set open or buys them by the role straight from the mint. As for telling difference between the zinc and cooper. Drop the coins one at a time on a solid surface. The cooper penny will make a sort of ringing sound and the zinc will make a different noise.