Originally Posted by eclipsisNA
Give this a try:
It could be that the side bolster is heavily worn and the white you're seeing are areas where the urethane or color is removed. I can't be positive without seeing it. If Leather Master Soft or Strong couldn't remove the marks, I don't know what could remove the marks without degrading the rest of the leather.
Cleaning with Leather Master Strong doesn't do anything. I can darken the leather a bit and mask the white marks with the leather master protectant and vital but it will eventually show white again after a few days.