Most jobs ever done by convicts as penal labor would be fundamentally unskilled, no? These are jobs with no expectation of unique talent or skill; with no lengthy on-the-job training; with no ability to fail at the job so badly that they would ever "fire" you. Jobs like "here's a pickaxe, start hitting rocks" or "sit here and pull down the stamper each time a license plate is in front of you" literally can't be done poorly — only done either efficiently or lazily.