At the risk of going too far into details, there is only about 1/8" clearance between the top face of the locating peg and the bottom of the scope tube. Impossible to get any bracket cradle in that space, therefore the pin was either sheared off or removed. The soldier who swapped the caps around could have unscrewed it since he clearly had the wherewithal to undo the eight cap screws. There is a hole drilled through the bronze boss and the screw itself, with a piece of wire inserted to keep the screw from coming loose, but I've undone a few and the wire just sheared off if enough force was used. The wire was too thin to push out with a punch so there was no choice in those cases. Pin was removed to polish the tubes.
Why put the caps back on? Force of habit? Initial thought was to render unusable, but not to destroy against possible ultimate recovery? How many think entirely rationally at such times after a good firefight and the trauma of impending surrender? The "thumb screws" and spring washers were apparently discarded where they could not be found, so that at least prevented reuse.