Mint Marks

From About.com:
Definition: A repunched mintmark, or RPM, is a coin that exhibits two or more images of the same mintmark, caused by errors in the punching of the mintmark onto the coin die (or very rarely, onto the hub.) Up until 1990, the U.S. Mint used to manually punch the mintmark into each individual coin die. Occasionally, due to human error, a die would get two or more punches of the same mintmark, sometimes in almost the same location, and sometimes at 90 or 180 degree rotations! Although the mint workers usually caught these defective dies before any coins were produced from them, on very occasions a die would strike coins with multiple impressions of the same mintmark letter. Such specimens are called repunched mintmarks, or RPMs. In general, they are very collectible.

It actually depends on the coin. They actually phased them out between 1985 and 1990 dropping another denomination each year with the cent being the last one to switch over to having the mintmark on the master hub.

Agreed, sorry didn't make that clear.