I think dust and dirt since its the inlet side. Wore away the plating that was probably marginal in the first place.
The marks on the crank are just chinese QC marks. There might be one piece of dirt there on the right side...
Caused by the skilled operater placing the crank wheels carefully in the parts bin when finishing each one.
As for the cracked piston? Maybe heat related from the alloy to alloy scuffing.
Or you just got the trifecta, A junk barrell, piston and crank...