1955 Poor man's Doubled Die
The 1955 "Poor man's Doubled Die" is actually not a Doubled Die at all.  The doubling that appears on the last 5
of the date was caused by die deterioration.  When die are overused, they begin erode.  This is called Die
Deterioration Doubling.   Not only is this
very common, but this same type of effect can be seen on other dates
like 1953.  The 1955 Poor man's doubled die is popular because dealers made it popular.  Some grading
companies actually certify this variety.