Hi, Paul!
It was likely plated in the 1950s or 60s, during the height of the coin collecting boom. Perhaps the 1955 penny represented a special year to the owner and thus why it was plated as memento -- or perhaps it was plated simply to recreate the novelty of the 1943 cent (something, amazingly, many people spent their free time doing). Presumably, the coin's owner passed the coin on to someone who probably had little, if any, numismatic knowledge about coins. That person likely spent the coin at face value, and it passed through the channels of circulation -- perhaps even being spent, mistakenly, as a dime at one point given its color.
I hope this sheds some light on your silvery penny!
All the best,
Josh