This is the adjustment screw that is found in the AMP regulator of FX.
This sets the pressure of the regulator. With many adjustments, the 2.5mm hex can sometimes wear out and it needs to be replaced.
This one comes without the two FX 19731 o-rings. You can find this here or below under the related products.
Note for FX Impact owners: The set screw for the first regulator (near the bottle) of the FX Impact is different. Please contact us if necessary.