S17 is an optional upgrade for K3 5 inch rigid casters that gives you a wider range of bolt hole pattern compatibility. With its 4″ x 4-1/2″ inch top plate and flexible bolt spacing of 2-5/8” x 3-5/8” Slotted To 3” x 3”, it opens the door to more dynamic mounting fits. This upgrade also increases the overall footprint, making the caster closer in size and usability to a Model 9.