I noticed the other day that the Clark Zapper has a small modification in the latest HIV/AIDS book for her Zapper. She added one more resistor to the circuit. Its value is 39k ohms. She calls it a Positive offset resistor. It used to keep the Zapper from a potential negative spike which she said was undesirable. You can read about it on page 506 of the book.
I added one to my Zapper today. It did not effect the frequency but the voltage was increased slightly, maybe .2 to .3 volts. I will try it in the next few days to see if I notice any difference with the new resistor.