[100IH]

If I understand the issue, there would be trim across the vertical interior side of the wood surface of the header. The trim consists of thick hard paper card material, thin cotton type of padding, and the interior cloth. All of this tucks into the metal strip at the bottom of the header. At that point, there should not be 3/8 inch step for the mirror mount. At that point the problem is; how do you find the holes if they're in the header- everything is covered up. Hope this helps.