Chevrolet CMP C15 cab 11 body 2A1

[Grant Bowker]

You are correct - the 3 holes in triangular pattern are for the rifle holder. (3 holes on cab 11 and 12, only 2 holes on cab 13 - the brackets are also different lengths so the hole pattern helps to get the correct bracket for each cab type)
I am not sure about the holes with "lift-the-dot" studs across the cab back, they might be to hang a bag to store the upper curtains and frames for the doors as well as a first aid kit but I am more used to seeing these clips hanging from the horizontal part of the angle instead of on the vertical. As well, I do not remember seeing the "lift-the-dot" studs" used for this. Most trucks I have seen use a bent metal clip hanging from the horizontal face of the angle.