I'll try to give the short answer. It all has to do with the hubbing of the dies, different details are added by what is called a hubbing. Say 1st hubbing is the bust, 2nd hubbing adds motto and LIBERTY, 3rd is date. At each point if die is rotated CW or CCW, shifting of the details happen causing a "doubled Die". MAJOR rotation happened on the 1955P & 1969S dies, Minor doubling happened on 1955P sister dies as well. More than one die was used and could be paired with multiple other hammer/anvil die pairs (obverse/reverse). The mint marks were added in the last stages of die preparation up to 1990. In 1990, the stamping process changed to the single squeeze method. That's the short answer...






















