This is the fire extinguisher mounting picture from the EPC, Mercedes part catalogue. I am not sure if it is this part - the shape is somewhat similar, but on the picture it may be a bolt rather than a hole in the upper part of this bracket. I am not sure how the hanger for fire extinguisher looked like - it may have a bolt to fit in the hole in the fire extinguisher to hold it.
If you have part numbers on some of the brackets that are not identified, with EPC you may figure out where they fit.