We purchased a 1969 MK11 last summer. She leaked around the older cockpit drains untill I replaced them with new black radiator hoses. I also think she was leaking around the same spot you described, but it was hard to tell cause it was always wet. My plan is for a bilge pump under the cockpit towards the stairs...where the drains come out..seems like the deepest part of the hull where water can collect...I want one there more for an emergency than any other reason. We keep a little hand pump on board and clean out near the floor board and keel bolts about once a week.
On your rudder..I seem to remember a site that had info on who owned the original molds etc. If I can find it I will write back.
Good Luck . we are in CT waiting for the snow to go away.
I just replaced the valve on this drain under the cockpit. You should have access thru the lazzerett. It's a tight place to work but much better than cutting holes. This is also 1971 Columbia 26 MK II.