What causes this type of wear pattern in the cylinder bore?

No history on the engine. Could be made up from bits and pieces, but I think it has been assembled a while with how dirty and grimy it is.
In this configuration, it is the number one cylinder and the dark trapezoidal pattern is on what would be the bottom as installed but at the top of the stroke for the piston.

Click to view attachment

Click to view attachment

Thanks,

Sean

Wild shot in the dark, it was parked outside for a while and that cylinder had an open valve. A little water and a little rust. Then someone started the engine after it sitting and you are left with a stain, possibly pitting. Again a shot in the dark

,give it a light sand with some wet and dry sand paper in that area,if it comes off and the barrel is up to spec you can hone and reuse it.
Good luck.