If it were my gun, I would fit a new leather seal. Carefully sanding it dry to an easy sliding fit in the cylinder. Drip a little oil onto the seal immediately before fitting, then allow the gun to stand for a while. I would definitely keep the flat headed screw and the brass inner washer, that fits over the top of a secondary leather inner washer, as all that is completely original. The impression of the transfer port on the head of the screw is quite normal. I see it in 75% of the guns I open up. If you need a drop or two more oil, it is easily introduced in through the loading tap, then left to soak into the seal. But you don't want to flood it.
Love these gun's !!
Lakey