Complex sentence: At that point the developing town was known as "Cockrill's Hill" and remained so until 1857 when the townspeople applied for a post office permit and changed the name of the town to "Milton" (the name of Cockrill's son).
Split into simpler sentences: At that point the developing town was known as "Cockrill's Hill". Until 1857, when the townspeople applied for a post office permit, and changing the name of the town to "Milton" (the name of Cockrill's son).