Any one have the part number for different pistons for the new HPI Shock part# 75194?

I bought these shocks but it only comes with 1 piston choice. They are suppose to be a closer tolerance piston than what was used in the older design but I have not been able to locate the part number for different piston #'s for this shock. Can anyone help?

Thanks

RC

#6818 pistons and their seems to be two different size pistons