I stuck a drill bit in there it measured 25mm deep and yes i think you are correct it must only be threaded like 6-8mm or something but i managed to tighten the bolt in further using some lubrication and backing it out as i go washing out any material. I used a 20mm long bolt (.78") When you subtract the .19" bracket and 2 washers you get about 12mm or .50" that is actually threaded into the hole. I felt better having at least this much holding on.

Hope this helps everyone