I agree I think the post is hollow, I only have one threaded portion for the mirror on the drivers side, between the mounting points for the windshield. I was hoping just to drill and tap the other passenger side but I am not sure how much metal there is in the post on the passenger side as the back side is covered and welded shut. I guess they could have welded a nut to the drivers side before they welded the inside cover on or the metal is thick enough to support the mirror in that area? thanks for your response appreciate it