What was the gospel of the sixth Undershaft?

I'm not sure about this.... I see no evidence of an Undershaft being described as the "sixth".

UNDERSHAFT [in towering excitement] It is the Undershaft inheritance. I shall hand on my torch to my daughter. She shall make my converts and preach my gospel.
CUSINS. What! Money and gunpowder!
UNDERSHAFT. Yes, money and gunpowder; freedom and power; command of life and command of death.