Puffs, Well, at least you now have a clear idea of the cause. The piston on my MZ150 has very sunken pins which makes it tricky to keep the rings located correctly when lowering the barrel. However I assumed that once inside the cylinder they would be adequate to stop the rings turning. So far, so good, but you have given me something else to worry about. As you have said previously, if the worst happens it will not be as expensive a fix as for you!
I believe its the original MZ piston.