Corse and Uriah thanks for the info.

The piston was not at BDC when the backplate was removed to the best of my knowledge, HOWEVER as the sleeve and the engine interior is in perfect condition and the broken pieces had not been smashed into many small pieces I believe that the piston must have come back down while I was removing the backplate and as Uriah said, broke the side of the piston. Cannot be anything else I guess!

If anyone is considering the Murnan tuned R01 it is very reliable with a great powerband, I would definitely recommend it