The piston pin of the Mercedes is of chrome nickel steel, and is retained in the piston by means of a set screw and cotter pin.
You couldn't pull a cotter pin with a pair of pliers if you knew what a cotter pin was.
Won't a cotter pin be cheaper and better in place of that nut?
On every bolt use a castellated nut and a cotter pin, or an ordinary nut and a lock washer, so the bolt will not work loose.
Or there may have been a little carelessness when erecting, and a cotter pin omitted.