George, the 1851 listings of United States Post Offices in Harrison County, Mississippi were as follows:
2) Flint Creek
3) Mississippi City - then the County Seat of Harrison County and now on the outskirts of Gulfport.
4) Pass Christian
As you know Stone County at this time period was also a part of Harrison County. Stone County was named after a Post War Governor and interestingly also a prisoner of war at Camp Chase. He would be the only Camp Chase prisoner to have a county named after him after the war.