Springfield 2-7/8" Heavy-Duty Adjustable Mainstay Package w/Tension Knob - 9" Base [1250456-LC]2 7 8" Heavy Duty Adjustable Mainstay Package with Tension Knob 9" Base Package includes 2 7 8 high strength rigid 4 keyed anodized aluminum air powered height adjustment pedestal (500 newton cylinder) with 9 round anodized base, and locking Trac Lock swivel & slide. 3 1 2 outer post with 2 7 8 inner post.