Your #3 piston looks almost like my #1 did on my 85 300D at 330k miles. Something was bouncing around in mine too, but not spherical like your marks. I am guessing the ball you are holding is the infamous "ball on a stick" that sits inside the pre-chamber. Mine couldn't have come out since my pre-chamber was intact, with just strike marks on the outside. I think my object was a chunk of piston, since the piston was missing a bit. However, I have also heard that my unique CA model with trap oxidizer can get pieces of ceramic back inside the engine, though the catalyst looks whole and secure. I heard a loud ball bearing rattle once that went away, drove the car another 5K miles, got another rattling and the engine soon siezed. Now have a 1983 engine in that has no blowby at all.