The 55th-59th regiments were generally formed from the GA State Troop regiments then serving along the coast. For example, Barkuloo's 57th was formed from Barkuloo's 2nd GST. Harkie's 55th was formed from the 1st GST.
In May 1862 Henry Williams placed ads in the newspapers saying he had been commissioned by the Sec. of War to raise a regiment of infantry. It was never raised. Perhaps the number 58 was reserved for William's regiment-to-be.
Henry Williams (1818-78) was the son of a mayor of Savannah, and a prominent lawyer who had been captain of Savannah's Pulaski Guards.