Firestone air bag W01-358-9966 comes with the plastic air bag base (Firestone piston 9583) included as part of the assembly. This Firestone air bag is used is used by multiple suspension and chassis companies. If Spartan does require an additional support plate you would have to contact them. If they are referring to the plastic base on the bottom of the air spring (the piston), it is included. Feel free to email us pictures (truckspring@truckspring.com) or call (1-800-358-4751) us to verify we are referring to the same support plate.