1995 Oldsmobile 88 thread and accurate measurements on brak

I tried to take measurements but the threads dont match. I have tried just about every bolt size I can get at hardware stores. I have been told from various mechanics that is a propietary bolt that is made only for certain cars. I called a dealership and to no avail they wont give me any info so I can duplicate it. I need a parts supplier or parts info. I tried asking this question once before but even though the answerer was really helpful I needed to do more research. I have been to salvage yards and still no luck.

This is great information. So just to make sure I understand cause when I look at my other light there are those "studs" that are threaded. These go through the housing and into the frame with nuts on the end. So realistically speaking I just need to use the thread size and find the proper length bolt. I am thinking these studs are made for Oldsmobile cars.