The caster wheel is composed of a caster_collision and caster_visual, both under the chassis link. The chassis origin (the center of the box) is 0.1 m on the Z axis with respect to the model origin. The caster collision and visual are exactly on top of each other. The sphere origin (its center) is -0.15 m in the X axis and -0.05 m in the Z axis from the link origin. If you want to know the caster's pose with respect to the model, just add this to the link's pose and you get [-0.15 0 -0.95 0 0 0].