Hi Barry, from this:

9.1 Ω (Stock): Gives standard 0.375V sensitivity.18 Ω: Lowers gain, raising sensitivity to 0.775V.27 Ω: Lowers gain further, raising sensitivity to 1.0V.36 Ω: Lowers gain to the ideal level for a 1.25V target sensitivity.

Resistor 13 on the Quad 306.