This is the cylinder which had lost some compression. The culprit was a worn or improperly installed stem seal on the exhaust valve. Ideally the compression will be restored after resurfacing and lapping the valves and seats. All new valve seals will be installed.
Valves, before and after surfacing. The small diameter valves with the fatter stems are sodium filled exhaust valves. These help move heat from the valve into the head where it can be absorbed and drawn away through the coolant galleys.