Yes, the Hex Nut # 936060054 that you referenced will fit the M6 x 25mm Carriage Bolt # 915-0625-54.
I spoke with my contact at Thule who said that they are metal (non-stainless steel) with a black powder coat glossy finish. My contact also said that he owns some Thule products that use this same nut and he has never seen any of them rust if that is what you are concerned about. view full answer...