This is how I think the spanner-nuts went on. (Edit, added the square-drive broached in the top, to aid loosening/tightening them)
In real life the part under the nut that holds the wrist-pin center, would be thinner, as it looks very much like a stamped part. In real life the connecting rod is drilled from big-end to little end. That center rib on the connecting rod, leaves material for drilling.
I might write the engine-museum, to see, if the piston can be taken out of the access-plug on the motor. I have read that the piston could be changed at sea; I don't know how else they could pull that off, as taking the heavy head off, with only hands to lift it would not be easy. There's a lot of parts that would have to come off otherwise.
I still suspect that the piston notches on top allow the piston to be turned in the crankcase, so it can be taken out of the access-opening. The whole thing would depend on the crank being made in such a way that it could be swung opposite the access opening.
