Looks like you got something, filing maybe, in the ring land and it did it's deed.
The knocking you heard was probably the wrist pin pivot to the piston skirt to the cylinder wall. Since there was more friction on one side of the piston than the other, the piston would slap in the cylinder.
Measure how deep the cut is and see if you can have it machined out.
Check the ring to land clearance all the way around that piston.
Bad break, sorry you had this happen.
Good thing you found it early.
