Hi Siafu. Welcome to Cointalk! It is most likely brass plating and it has significantly DECREASED the value of the penny to a collector anyway. It's still worth 1 cent.
Me thinks siafu might hail from the good old U.K. I don't know anyone else that uses the term "jacketed" except the Brits. :goof:
1st Welcome To cointalk! looks like the 55 may have environmental damage. * 1955 the US Mint was minting a lot of other Country's coinage,maybe struck on wrong planchet??? a better photo would help too