it could be just a stain, but i've also seen marks like that from excessive piston/ wall clearance. check it for size with a bore gauge compared to the rest of the cylinder. i bet you find it's a big wear spot from the piston rocking in the bore when the crank/ rod piston comes around from BDC
that area is large enough that it would take over a pint of liquid to spread out that far. that would be enough to hydrolock the motor.